1 files changed, 347 insertions, 162 deletions

diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
index f2bf1f73d468..d677fa9ca650 100644
--- a/arch/x86/lguest/boot.c
+++ b/arch/x86/lguest/boot.c
@@ -22,7 +22,8 @@
  *
  * So how does the kernel know it's a Guest?  We'll see that later, but let's
  * just say that we end up here where we replace the native functions various
- * "paravirt" structures with our Guest versions, then boot like normal. :*/
+ * "paravirt" structures with our Guest versions, then boot like normal.
+:*/
 
 /*
  * Copyright (C) 2006, Rusty Russell <rusty@rustcorp.com.au> IBM Corporation.
@@ -74,7 +75,8 @@
  *
  * The Guest in our tale is a simple creature: identical to the Host but
  * behaving in simplified but equivalent ways.  In particular, the Guest is the
- * same kernel as the Host (or at least, built from the same source code). :*/
+ * same kernel as the Host (or at least, built from the same source code).
+:*/
 
 struct lguest_data lguest_data = {
         .hcall_status = { [0 ... LHCALL_RING_SIZE-1] = 0xFF },
@@ -85,7 +87,8 @@ struct lguest_data lguest_data = {
         .syscall_vec = SYSCALL_VECTOR,
 };
 
-/*G:037 async_hcall() is pretty simple: I'm quite proud of it really.  We have a
+/*G:037
+ * async_hcall() is pretty simple: I'm quite proud of it really.  We have a
  * ring buffer of stored hypercalls which the Host will run though next time we
  * do a normal hypercall.  Each entry in the ring has 5 slots for the hypercall
  * arguments, and a "hcall_status" word which is 0 if the call is ready to go,
@@ -94,7 +97,8 @@ struct lguest_data lguest_data = {
  * If we come around to a slot which hasn't been finished, then the table is
  * full and we just make the hypercall directly.  This has the nice side
  * effect of causing the Host to run all the stored calls in the ring buffer
- * which empties it for next time! */
+ * which empties it for next time!
+ */
 static void async_hcall(unsigned long call, unsigned long arg1,
                         unsigned long arg2, unsigned long arg3,
                         unsigned long arg4)
@@ -103,9 +107,11 @@ static void async_hcall(unsigned long call, unsigned long arg1,
         static unsigned int next_call;
         unsigned long flags;
 
-        /* Disable interrupts if not already disabled: we don't want an
+        /*
+         * Disable interrupts if not already disabled: we don't want an
          * interrupt handler making a hypercall while we're already doing
-         * one! */
+         * one!
+         */
         local_irq_save(flags);
         if (lguest_data.hcall_status[next_call] != 0xFF) {
                 /* Table full, so do normal hcall which will flush table. */
@@ -125,8 +131,9 @@ static void async_hcall(unsigned long call, unsigned long arg1,
         local_irq_restore(flags);
 }
 
-/*G:035 Notice the lazy_hcall() above, rather than hcall().  This is our first
- * real optimization trick!
+/*G:035
+ * Notice the lazy_hcall() above, rather than hcall().  This is our first real
+ * optimization trick!
  *
  * When lazy_mode is set, it means we're allowed to defer all hypercalls and do
  * them as a batch when lazy_mode is eventually turned off.  Because hypercalls
@@ -136,7 +143,8 @@ static void async_hcall(unsigned long call, unsigned long arg1,
  * lguest_leave_lazy_mode().
  *
  * So, when we're in lazy mode, we call async_hcall() to store the call for
- * future processing: */
+ * future processing:
+ */
 static void lazy_hcall1(unsigned long call,
                        unsigned long arg1)
 {
@@ -146,6 +154,7 @@ static void lazy_hcall1(unsigned long call,
                 async_hcall(call, arg1, 0, 0, 0);
 }
 
+/* You can imagine what lazy_hcall2, 3 and 4 look like. :*/
 static void lazy_hcall2(unsigned long call,
                        unsigned long arg1,
                        unsigned long arg2)
@@ -181,8 +190,10 @@ static void lazy_hcall4(unsigned long call,
 }
 #endif
 
-/* When lazy mode is turned off reset the per-cpu lazy mode variable and then
- * issue the do-nothing hypercall to flush any stored calls. */
+/*G:036
+ * When lazy mode is turned off reset the per-cpu lazy mode variable and then
+ * issue the do-nothing hypercall to flush any stored calls.
+:*/
 static void lguest_leave_lazy_mmu_mode(void)
 {
         kvm_hypercall0(LHCALL_FLUSH_ASYNC);
@@ -208,9 +219,11 @@ static void lguest_end_context_switch(struct task_struct *next)
  * check there before it tries to deliver an interrupt.
  */
 
-/* save_flags() is expected to return the processor state (ie. "flags").  The
+/*
+ * save_flags() is expected to return the processor state (ie. "flags").  The
  * flags word contains all kind of stuff, but in practice Linux only cares
- * about the interrupt flag.  Our "save_flags()" just returns that. */
+ * about the interrupt flag.  Our "save_flags()" just returns that.
+ */
 static unsigned long save_fl(void)
 {
         return lguest_data.irq_enabled;
@@ -222,13 +235,15 @@ static void irq_disable(void)
         lguest_data.irq_enabled = 0;
 }
 
-/* Let's pause a moment.  Remember how I said these are called so often?
+/*
+ * Let's pause a moment.  Remember how I said these are called so often?
  * Jeremy Fitzhardinge optimized them so hard early in 2009 that he had to
  * break some rules.  In particular, these functions are assumed to save their
  * own registers if they need to: normal C functions assume they can trash the
  * eax register.  To use normal C functions, we use
  * PV_CALLEE_SAVE_REGS_THUNK(), which pushes %eax onto the stack, calls the
- * C function, then restores it. */
+ * C function, then restores it.
+ */
 PV_CALLEE_SAVE_REGS_THUNK(save_fl);
 PV_CALLEE_SAVE_REGS_THUNK(irq_disable);
 /*:*/
@@ -237,18 +252,18 @@ PV_CALLEE_SAVE_REGS_THUNK(irq_disable);
 extern void lg_irq_enable(void);
 extern void lg_restore_fl(unsigned long flags);
 
-/*M:003 Note that we don't check for outstanding interrupts when we re-enable
- * them (or when we unmask an interrupt).  This seems to work for the moment,
- * since interrupts are rare and we'll just get the interrupt on the next timer
- * tick, but now we can run with CONFIG_NO_HZ, we should revisit this.  One way
- * would be to put the "irq_enabled" field in a page by itself, and have the
- * Host write-protect it when an interrupt comes in when irqs are disabled.
- * There will then be a page fault as soon as interrupts are re-enabled.
+/*M:003
+ * We could be more efficient in our checking of outstanding interrupts, rather
+ * than using a branch.  One way would be to put the "irq_enabled" field in a
+ * page by itself, and have the Host write-protect it when an interrupt comes
+ * in when irqs are disabled.  There will then be a page fault as soon as
+ * interrupts are re-enabled.
  *
  * A better method is to implement soft interrupt disable generally for x86:
  * instead of disabling interrupts, we set a flag.  If an interrupt does come
  * in, we then disable them for real.  This is uncommon, so we could simply use
- * a hypercall for interrupt control and not worry about efficiency. :*/
+ * a hypercall for interrupt control and not worry about efficiency.
+:*/
 
 /*G:034
  * The Interrupt Descriptor Table (IDT).
@@ -261,10 +276,12 @@ extern void lg_restore_fl(unsigned long flags);
 static void lguest_write_idt_entry(gate_desc *dt,
                                    int entrynum, const gate_desc *g)
 {
-        /* The gate_desc structure is 8 bytes long: we hand it to the Host in
+        /*
+         * The gate_desc structure is 8 bytes long: we hand it to the Host in
          * two 32-bit chunks.  The whole 32-bit kernel used to hand descriptors
          * around like this; typesafety wasn't a big concern in Linux's early
-         * years. */
+         * years.
+         */
         u32 *desc = (u32 *)g;
         /* Keep the local copy up to date. */
         native_write_idt_entry(dt, entrynum, g);
@@ -272,9 +289,11 @@ static void lguest_write_idt_entry(gate_desc *dt,
         kvm_hypercall3(LHCALL_LOAD_IDT_ENTRY, entrynum, desc[0], desc[1]);
 }
 
-/* Changing to a different IDT is very rare: we keep the IDT up-to-date every
+/*
+ * Changing to a different IDT is very rare: we keep the IDT up-to-date every
  * time it is written, so we can simply loop through all entries and tell the
- * Host about them. */
+ * Host about them.
+ */
 static void lguest_load_idt(const struct desc_ptr *desc)
 {
         unsigned int i;
@@ -305,9 +324,11 @@ static void lguest_load_gdt(const struct desc_ptr *desc)
                 kvm_hypercall3(LHCALL_LOAD_GDT_ENTRY, i, gdt[i].a, gdt[i].b);
 }
 
-/* For a single GDT entry which changes, we do the lazy thing: alter our GDT,
+/*
+ * For a single GDT entry which changes, we do the lazy thing: alter our GDT,
  * then tell the Host to reload the entire thing.  This operation is so rare
- * that this naive implementation is reasonable. */
+ * that this naive implementation is reasonable.
+ */
 static void lguest_write_gdt_entry(struct desc_struct *dt, int entrynum,
                                    const void *desc, int type)
 {
@@ -317,29 +338,36 @@ static void lguest_write_gdt_entry(struct desc_struct *dt, int entrynum,
                        dt[entrynum].a, dt[entrynum].b);
 }
 
-/* OK, I lied.  There are three "thread local storage" GDT entries which change
+/*
+ * OK, I lied.  There are three "thread local storage" GDT entries which change
  * on every context switch (these three entries are how glibc implements
- * __thread variables).  So we have a hypercall specifically for this case. */
+ * __thread variables).  So we have a hypercall specifically for this case.
+ */
 static void lguest_load_tls(struct thread_struct *t, unsigned int cpu)
 {
-        /* There's one problem which normal hardware doesn't have: the Host
+        /*
+         * There's one problem which normal hardware doesn't have: the Host
          * can't handle us removing entries we're currently using.  So we clear
-         * the GS register here: if it's needed it'll be reloaded anyway. */
+         * the GS register here: if it's needed it'll be reloaded anyway.
+         */
         lazy_load_gs(0);
         lazy_hcall2(LHCALL_LOAD_TLS, __pa(&t->tls_array), cpu);
 }
 
-/*G:038 That's enough excitement for now, back to ploughing through each of
- * the different pv_ops structures (we're about 1/3 of the way through).
+/*G:038
+ * That's enough excitement for now, back to ploughing through each of the
+ * different pv_ops structures (we're about 1/3 of the way through).
  *
  * This is the Local Descriptor Table, another weird Intel thingy.  Linux only
  * uses this for some strange applications like Wine.  We don't do anything
- * here, so they'll get an informative and friendly Segmentation Fault. */
+ * here, so they'll get an informative and friendly Segmentation Fault.
+ */
 static void lguest_set_ldt(const void *addr, unsigned entries)
 {
 }
 
-/* This loads a GDT entry into the "Task Register": that entry points to a
+/*
+ * This loads a GDT entry into the "Task Register": that entry points to a
  * structure called the Task State Segment.  Some comments scattered though the
  * kernel code indicate that this used for task switching in ages past, along
  * with blood sacrifice and astrology.
@@ -347,19 +375,21 @@ static void lguest_set_ldt(const void *addr, unsigned entries)
  * Now there's nothing interesting in here that we don't get told elsewhere.
  * But the native version uses the "ltr" instruction, which makes the Host
  * complain to the Guest about a Segmentation Fault and it'll oops.  So we
- * override the native version with a do-nothing version. */
+ * override the native version with a do-nothing version.
+ */
 static void lguest_load_tr_desc(void)
 {
 }
 
-/* The "cpuid" instruction is a way of querying both the CPU identity
+/*
+ * The "cpuid" instruction is a way of querying both the CPU identity
  * (manufacturer, model, etc) and its features.  It was introduced before the
  * Pentium in 1993 and keeps getting extended by both Intel, AMD and others.
  * As you might imagine, after a decade and a half this treatment, it is now a
  * giant ball of hair.  Its entry in the current Intel manual runs to 28 pages.
  *
  * This instruction even it has its own Wikipedia entry.  The Wikipedia entry
- * has been translated into 4 languages.  I am not making this up!
+ * has been translated into 5 languages.  I am not making this up!
  *
  * We could get funky here and identify ourselves as "GenuineLguest", but
  * instead we just use the real "cpuid" instruction.  Then I pretty much turned
@@ -371,7 +401,8 @@ static void lguest_load_tr_desc(void)
  * Replacing the cpuid so we can turn features off is great for the kernel, but
  * anyone (including userspace) can just use the raw "cpuid" instruction and
  * the Host won't even notice since it isn't privileged.  So we try not to get
- * too worked up about it. */
+ * too worked up about it.
+ */
 static void lguest_cpuid(unsigned int *ax, unsigned int *bx,
                          unsigned int *cx, unsigned int *dx)
 {
@@ -379,43 +410,63 @@ static void lguest_cpuid(unsigned int *ax, unsigned int *bx,
 
         native_cpuid(ax, bx, cx, dx);
         switch (function) {
-        case 0: /* ID and highest CPUID.  Futureproof a little by sticking to
-                 * older ones. */
+        /*
+         * CPUID 0 gives the highest legal CPUID number (and the ID string).
+         * We futureproof our code a little by sticking to known CPUID values.
+         */
+        case 0:
                 if (*ax > 5)
                         *ax = 5;
                 break;
-        case 1: /* Basic feature request. */
-                /* We only allow kernel to see SSE3, CMPXCHG16B and SSSE3 */
+
+        /*
+         * CPUID 1 is a basic feature request.
+         *
+         * CX: we only allow kernel to see SSE3, CMPXCHG16B and SSSE3
+         * DX: SSE, SSE2, FXSR, MMX, CMOV, CMPXCHG8B, TSC, FPU and PAE.
+         */
+        case 1:
                 *cx &= 0x00002201;
-                /* SSE, SSE2, FXSR, MMX, CMOV, CMPXCHG8B, TSC, FPU, PAE. */
                 *dx &= 0x07808151;
-                /* The Host can do a nice optimization if it knows that the
+                /*
+                 * The Host can do a nice optimization if it knows that the
                  * kernel mappings (addresses above 0xC0000000 or whatever
                  * PAGE_OFFSET is set to) haven't changed.  But Linux calls
                  * flush_tlb_user() for both user and kernel mappings unless
-                 * the Page Global Enable (PGE) feature bit is set. */
+                 * the Page Global Enable (PGE) feature bit is set.
+                 */
                 *dx |= 0x00002000;
-                /* We also lie, and say we're family id 5.  6 or greater
+                /*
+                 * We also lie, and say we're family id 5.  6 or greater
                  * leads to a rdmsr in early_init_intel which we can't handle.
-                 * Family ID is returned as bits 8-12 in ax. */
+                 * Family ID is returned as bits 8-12 in ax.
+                 */
                 *ax &= 0xFFFFF0FF;
                 *ax |= 0x00000500;
                 break;
+        /*
+         * 0x80000000 returns the highest Extended Function, so we futureproof
+         * like we do above by limiting it to known fields.
+         */
         case 0x80000000:
-                /* Futureproof this a little: if they ask how much extended
-                 * processor information there is, limit it to known fields. */
                 if (*ax > 0x80000008)
                         *ax = 0x80000008;
                 break;
+
+        /*
+         * PAE systems can mark pages as non-executable.  Linux calls this the
+         * NX bit.  Intel calls it XD (eXecute Disable), AMD EVP (Enhanced
+         * Virus Protection).  We just switch turn if off here, since we don't
+         * support it.
+         */
         case 0x80000001:
-                /* Here we should fix nx cap depending on host. */
-                /* For this version of PAE, we just clear NX bit. */
                 *dx &= ~(1 << 20);
                 break;
         }
 }
 
-/* Intel has four control registers, imaginatively named cr0, cr2, cr3 and cr4.
+/*
+ * Intel has four control registers, imaginatively named cr0, cr2, cr3 and cr4.
  * I assume there's a cr1, but it hasn't bothered us yet, so we'll not bother
  * it.  The Host needs to know when the Guest wants to change them, so we have
  * a whole series of functions like read_cr0() and write_cr0().
@@ -430,7 +481,8 @@ static void lguest_cpuid(unsigned int *ax, unsigned int *bx,
  * name like "FPUTRAP bit" be a little less cryptic?
  *
  * We store cr0 locally because the Host never changes it.  The Guest sometimes
- * wants to read it and we'd prefer not to bother the Host unnecessarily. */
+ * wants to read it and we'd prefer not to bother the Host unnecessarily.
+ */
 static unsigned long current_cr0;
 static void lguest_write_cr0(unsigned long val)
 {
@@ -443,18 +495,22 @@ static unsigned long lguest_read_cr0(void)
         return current_cr0;
 }
 
-/* Intel provided a special instruction to clear the TS bit for people too cool
+/*
+ * Intel provided a special instruction to clear the TS bit for people too cool
  * to use write_cr0() to do it.  This "clts" instruction is faster, because all
- * the vowels have been optimized out. */
+ * the vowels have been optimized out.
+ */
 static void lguest_clts(void)
 {
         lazy_hcall1(LHCALL_TS, 0);
         current_cr0 &= ~X86_CR0_TS;
 }
 
-/* cr2 is the virtual address of the last page fault, which the Guest only ever
+/*
+ * cr2 is the virtual address of the last page fault, which the Guest only ever
  * reads.  The Host kindly writes this into our "struct lguest_data", so we
- * just read it out of there. */
+ * just read it out of there.
+ */
 static unsigned long lguest_read_cr2(void)
 {
         return lguest_data.cr2;
@@ -463,10 +519,12 @@ static unsigned long lguest_read_cr2(void)
 /* See lguest_set_pte() below. */
 static bool cr3_changed = false;
 
-/* cr3 is the current toplevel pagetable page: the principle is the same as
+/*
+ * cr3 is the current toplevel pagetable page: the principle is the same as
  * cr0.  Keep a local copy, and tell the Host when it changes.  The only
  * difference is that our local copy is in lguest_data because the Host needs
- * to set it upon our initial hypercall. */
+ * to set it upon our initial hypercall.
+ */
 static void lguest_write_cr3(unsigned long cr3)
 {
         lguest_data.pgdir = cr3;
@@ -511,7 +569,7 @@ static void lguest_write_cr4(unsigned long val)
  * cr3 ---> +---------+
  *          |      --------->+---------+
  *          |         |      | PADDR1  |
- *        Top-level   |      | PADDR2  |
+ *        Mid-level   |      | PADDR2  |
  *        (PMD) page  |      |         |
  *          |         |    Lower-level |
  *          |         |    (PTE) page  |
@@ -531,21 +589,62 @@ static void lguest_write_cr4(unsigned long val)
  *    Index into top     Index into second      Offset within page
  *  page directory page    pagetable page
  *
- * The kernel spends a lot of time changing both the top-level page directory
- * and lower-level pagetable pages.  The Guest doesn't know physical addresses,
- * so while it maintains these page tables exactly like normal, it also needs
- * to keep the Host informed whenever it makes a change: the Host will create
- * the real page tables based on the Guests'.
+ * Now, unfortunately, this isn't the whole story: Intel added Physical Address
+ * Extension (PAE) to allow 32 bit systems to use 64GB of memory (ie. 36 bits).
+ * These are held in 64-bit page table entries, so we can now only fit 512
+ * entries in a page, and the neat three-level tree breaks down.
+ *
+ * The result is a four level page table:
+ *
+ * cr3 --> [ 4 Upper  ]
+ *         [   Level  ]
+ *         [  Entries ]
+ *         [(PUD Page)]---> +---------+
+ *                          |      --------->+---------+
+ *                          |         |      | PADDR1  |
+ *                        Mid-level   |      | PADDR2  |
+ *                        (PMD) page  |      |         |
+ *                          |         |    Lower-level |
+ *                          |         |    (PTE) page  |
+ *                          |         |      |         |
+ *                            ....               ....
+ *
+ *
+ * And the virtual address is decoded as:
+ *
+ *         1 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
+ *      |<-2->|<--- 9 bits ---->|<---- 9 bits --->|<------ 12 bits ------>|
+ * Index into    Index into mid    Index into lower    Offset within page
+ * top entries   directory page     pagetable page
+ *
+ * It's too hard to switch between these two formats at runtime, so Linux only
+ * supports one or the other depending on whether CONFIG_X86_PAE is set.  Many
+ * distributions turn it on, and not just for people with silly amounts of
+ * memory: the larger PTE entries allow room for the NX bit, which lets the
+ * kernel disable execution of pages and increase security.
+ *
+ * This was a problem for lguest, which couldn't run on these distributions;
+ * then Matias Zabaljauregui figured it all out and implemented it, and only a
+ * handful of puppies were crushed in the process!
+ *
+ * Back to our point: the kernel spends a lot of time changing both the
+ * top-level page directory and lower-level pagetable pages.  The Guest doesn't
+ * know physical addresses, so while it maintains these page tables exactly
+ * like normal, it also needs to keep the Host informed whenever it makes a
+ * change: the Host will create the real page tables based on the Guests'.
  */
 
-/* The Guest calls this to set a second-level entry (pte), ie. to map a page
- * into a process' address space.  We set the entry then tell the Host the
- * toplevel and address this corresponds to.  The Guest uses one pagetable per
- * process, so we need to tell the Host which one we're changing (mm->pgd). */
+/*
+ * The Guest calls this after it has set a second-level entry (pte), ie. to map
+ * a page into a process' address space.  Wetell the Host the toplevel and
+ * address this corresponds to.  The Guest uses one pagetable per process, so
+ * we need to tell the Host which one we're changing (mm->pgd).
+ */
 static void lguest_pte_update(struct mm_struct *mm, unsigned long addr,
                                pte_t *ptep)
 {
 #ifdef CONFIG_X86_PAE
+        /* PAE needs to hand a 64 bit page table entry, so it uses two args. */
         lazy_hcall4(LHCALL_SET_PTE, __pa(mm->pgd), addr,
                     ptep->pte_low, ptep->pte_high);
 #else
@@ -553,6 +652,7 @@ static void lguest_pte_update(struct mm_struct *mm, unsigned long addr,
 #endif
 }
 
+/* This is the "set and update" combo-meal-deal version. */
 static void lguest_set_pte_at(struct mm_struct *mm, unsigned long addr,
                               pte_t *ptep, pte_t pteval)
 {
@@ -560,10 +660,13 @@ static void lguest_set_pte_at(struct mm_struct *mm, unsigned long addr,
         lguest_pte_update(mm, addr, ptep);
 }
 
-/* The Guest calls lguest_set_pud to set a top-level entry and lguest_set_pmd
+/*
+ * The Guest calls lguest_set_pud to set a top-level entry and lguest_set_pmd
  * to set a middle-level entry when PAE is activated.
+ *
  * Again, we set the entry then tell the Host which page we changed,
- * and the index of the entry we changed. */
+ * and the index of the entry we changed.
+ */
 #ifdef CONFIG_X86_PAE
 static void lguest_set_pud(pud_t *pudp, pud_t pudval)
 {
@@ -582,8 +685,7 @@ static void lguest_set_pmd(pmd_t *pmdp, pmd_t pmdval)
 }
 #else
 
-/* The Guest calls lguest_set_pmd to set a top-level entry when PAE is not
- * activated. */
+/* The Guest calls lguest_set_pmd to set a top-level entry when !PAE. */
 static void lguest_set_pmd(pmd_t *pmdp, pmd_t pmdval)
 {
         native_set_pmd(pmdp, pmdval);
@@ -592,7 +694,8 @@ static void lguest_set_pmd(pmd_t *pmdp, pmd_t pmdval)
 }
 #endif
 
-/* There are a couple of legacy places where the kernel sets a PTE, but we
+/*
+ * There are a couple of legacy places where the kernel sets a PTE, but we
  * don't know the top level any more.  This is useless for us, since we don't
  * know which pagetable is changing or what address, so we just tell the Host
  * to forget all of them.  Fortunately, this is very rare.
@@ -600,7 +703,8 @@ static void lguest_set_pmd(pmd_t *pmdp, pmd_t pmdval)
  * ... except in early boot when the kernel sets up the initial pagetables,
  * which makes booting astonishingly slow: 1.83 seconds!  So we don't even tell
  * the Host anything changed until we've done the first page table switch,
- * which brings boot back to 0.25 seconds. */
+ * which brings boot back to 0.25 seconds.
+ */
 static void lguest_set_pte(pte_t *ptep, pte_t pteval)
 {
         native_set_pte(ptep, pteval);
@@ -609,6 +713,11 @@ static void lguest_set_pte(pte_t *ptep, pte_t pteval)
 }
 
 #ifdef CONFIG_X86_PAE
+/*
+ * With 64-bit PTE values, we need to be careful setting them: if we set 32
+ * bits at a time, the hardware could see a weird half-set entry.  These
+ * versions ensure we update all 64 bits at once.
+ */
 static void lguest_set_pte_atomic(pte_t *ptep, pte_t pte)
 {
         native_set_pte_atomic(ptep, pte);
@@ -616,19 +725,21 @@ static void lguest_set_pte_atomic(pte_t *ptep, pte_t pte)
                 lazy_hcall1(LHCALL_FLUSH_TLB, 1);
 }
 
-void lguest_pte_clear(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
+static void lguest_pte_clear(struct mm_struct *mm, unsigned long addr,
+                             pte_t *ptep)
 {
         native_pte_clear(mm, addr, ptep);
         lguest_pte_update(mm, addr, ptep);
 }
 
-void lguest_pmd_clear(pmd_t *pmdp)
+static void lguest_pmd_clear(pmd_t *pmdp)
 {
         lguest_set_pmd(pmdp, __pmd(0));
 }
 #endif
 
-/* Unfortunately for Lguest, the pv_mmu_ops for page tables were based on
+/*
+ * Unfortunately for Lguest, the pv_mmu_ops for page tables were based on
  * native page table operations.  On native hardware you can set a new page
  * table entry whenever you want, but if you want to remove one you have to do
  * a TLB flush (a TLB is a little cache of page table entries kept by the CPU).
@@ -637,24 +748,29 @@ void lguest_pmd_clear(pmd_t *pmdp)
  * called when a valid entry is written, not when it's removed (ie. marked not
  * present).  Instead, this is where we come when the Guest wants to remove a
  * page table entry: we tell the Host to set that entry to 0 (ie. the present
- * bit is zero). */
+ * bit is zero).
+ */
 static void lguest_flush_tlb_single(unsigned long addr)
 {
         /* Simply set it to zero: if it was not, it will fault back in. */
         lazy_hcall3(LHCALL_SET_PTE, lguest_data.pgdir, addr, 0);
 }
 
-/* This is what happens after the Guest has removed a large number of entries.
+/*
+ * This is what happens after the Guest has removed a large number of entries.
  * This tells the Host that any of the page table entries for userspace might
- * have changed, ie. virtual addresses below PAGE_OFFSET. */
+ * have changed, ie. virtual addresses below PAGE_OFFSET.
+ */
 static void lguest_flush_tlb_user(void)
 {
         lazy_hcall1(LHCALL_FLUSH_TLB, 0);
 }
 
-/* This is called when the kernel page tables have changed.  That's not very
+/*
+ * This is called when the kernel page tables have changed.  That's not very
  * common (unless the Guest is using highmem, which makes the Guest extremely
- * slow), so it's worth separating this from the user flushing above. */
+ * slow), so it's worth separating this from the user flushing above.
+ */
 static void lguest_flush_tlb_kernel(void)
 {
         lazy_hcall1(LHCALL_FLUSH_TLB, 1);
@@ -691,26 +807,38 @@ static struct irq_chip lguest_irq_controller = {
         .unmask         = enable_lguest_irq,
 };
 
-/* This sets up the Interrupt Descriptor Table (IDT) entry for each hardware
+/*
+ * This sets up the Interrupt Descriptor Table (IDT) entry for each hardware
  * interrupt (except 128, which is used for system calls), and then tells the
  * Linux infrastructure that each interrupt is controlled by our level-based
- * lguest interrupt controller. */
+ * lguest interrupt controller.
+ */
 static void __init lguest_init_IRQ(void)
 {
         unsigned int i;
 
         for (i = FIRST_EXTERNAL_VECTOR; i < NR_VECTORS; i++) {
-                /* Some systems map "vectors" to interrupts weirdly.  Lguest has
-                 * a straightforward 1 to 1 mapping, so force that here. */
+                /* Some systems map "vectors" to interrupts weirdly.  Not us! */
                 __get_cpu_var(vector_irq)[i] = i - FIRST_EXTERNAL_VECTOR;
                 if (i != SYSCALL_VECTOR)
                         set_intr_gate(i, interrupt[i - FIRST_EXTERNAL_VECTOR]);
         }
-        /* This call is required to set up for 4k stacks, where we have
-         * separate stacks for hard and soft interrupts. */
+
+        /*
+         * This call is required to set up for 4k stacks, where we have
+         * separate stacks for hard and soft interrupts.
+         */
         irq_ctx_init(smp_processor_id());
 }
 
+/*
+ * With CONFIG_SPARSE_IRQ, interrupt descriptors are allocated as-needed, so
+ * rather than set them in lguest_init_IRQ we are called here every time an
+ * lguest device needs an interrupt.
+ *
+ * FIXME: irq_to_desc_alloc_node() can fail due to lack of memory, we should
+ * pass that up!
+ */
 void lguest_setup_irq(unsigned int irq)
 {
         irq_to_desc_alloc_node(irq, 0);
@@ -729,31 +857,39 @@ static unsigned long lguest_get_wallclock(void)
         return lguest_data.time.tv_sec;
 }
 
-/* The TSC is an Intel thing called the Time Stamp Counter.  The Host tells us
+/*
+ * The TSC is an Intel thing called the Time Stamp Counter.  The Host tells us
  * what speed it runs at, or 0 if it's unusable as a reliable clock source.
  * This matches what we want here: if we return 0 from this function, the x86
- * TSC clock will give up and not register itself. */
+ * TSC clock will give up and not register itself.
+ */
 static unsigned long lguest_tsc_khz(void)
 {
         return lguest_data.tsc_khz;
 }
 
-/* If we can't use the TSC, the kernel falls back to our lower-priority
- * "lguest_clock", where we read the time value given to us by the Host. */
+/*
+ * If we can't use the TSC, the kernel falls back to our lower-priority
+ * "lguest_clock", where we read the time value given to us by the Host.
+ */
 static cycle_t lguest_clock_read(struct clocksource *cs)
 {
         unsigned long sec, nsec;
 
-        /* Since the time is in two parts (seconds and nanoseconds), we risk
+        /*
+         * Since the time is in two parts (seconds and nanoseconds), we risk
          * reading it just as it's changing from 99 & 0.999999999 to 100 and 0,
          * and getting 99 and 0.  As Linux tends to come apart under the stress
-         * of time travel, we must be careful: */
+         * of time travel, we must be careful:
+         */
         do {
                 /* First we read the seconds part. */
                 sec = lguest_data.time.tv_sec;
-                /* This read memory barrier tells the compiler and the CPU that
+                /*
+                 * This read memory barrier tells the compiler and the CPU that
                  * this can't be reordered: we have to complete the above
-                 * before going on. */
+                 * before going on.
+                 */
                 rmb();
                 /* Now we read the nanoseconds part. */
                 nsec = lguest_data.time.tv_nsec;
@@ -777,9 +913,11 @@ static struct clocksource lguest_clock = {
         .flags          = CLOCK_SOURCE_IS_CONTINUOUS,
 };
 
-/* We also need a "struct clock_event_device": Linux asks us to set it to go
+/*
+ * We also need a "struct clock_event_device": Linux asks us to set it to go
  * off some time in the future.  Actually, James Morris figured all this out, I
- * just applied the patch. */
+ * just applied the patch.
+ */
 static int lguest_clockevent_set_next_event(unsigned long delta,
                                            struct clock_event_device *evt)
 {
@@ -829,8 +967,10 @@ static struct clock_event_device lguest_clockevent = {
         .max_delta_ns           = LG_CLOCK_MAX_DELTA,
 };
 
-/* This is the Guest timer interrupt handler (hardware interrupt 0).  We just
- * call the clockevent infrastructure and it does whatever needs doing. */
+/*
+ * This is the Guest timer interrupt handler (hardware interrupt 0).  We just
+ * call the clockevent infrastructure and it does whatever needs doing.
+ */
 static void lguest_time_irq(unsigned int irq, struct irq_desc *desc)
 {
         unsigned long flags;
@@ -841,10 +981,12 @@ static void lguest_time_irq(unsigned int irq, struct irq_desc *desc)
         local_irq_restore(flags);
 }
 
-/* At some point in the boot process, we get asked to set up our timing
+/*
+ * At some point in the boot process, we get asked to set up our timing
  * infrastructure.  The kernel doesn't expect timer interrupts before this, but
  * we cleverly initialized the "blocked_interrupts" field of "struct
- * lguest_data" so that timer interrupts were blocked until now. */
+ * lguest_data" so that timer interrupts were blocked until now.
+ */
 static void lguest_time_init(void)
 {
         /* Set up the timer interrupt (0) to go to our simple timer routine */
@@ -868,14 +1010,16 @@ static void lguest_time_init(void)
  * to work.  They're pretty simple.
  */
 
-/* The Guest needs to tell the Host what stack it expects traps to use.  For
+/*
+ * The Guest needs to tell the Host what stack it expects traps to use.  For
  * native hardware, this is part of the Task State Segment mentioned above in
  * lguest_load_tr_desc(), but to help hypervisors there's this special call.
  *
  * We tell the Host the segment we want to use (__KERNEL_DS is the kernel data
  * segment), the privilege level (we're privilege level 1, the Host is 0 and
  * will not tolerate us trying to use that), the stack pointer, and the number
- * of pages in the stack. */
+ * of pages in the stack.
+ */
 static void lguest_load_sp0(struct tss_struct *tss,
                             struct thread_struct *thread)
 {
@@ -889,7 +1033,8 @@ static void lguest_set_debugreg(int regno, unsigned long value)
         /* FIXME: Implement */
 }
 
-/* There are times when the kernel wants to make sure that no memory writes are
+/*
+ * There are times when the kernel wants to make sure that no memory writes are
  * caught in the cache (that they've all reached real hardware devices).  This
  * doesn't matter for the Guest which has virtual hardware.
  *
@@ -903,11 +1048,13 @@ static void lguest_wbinvd(void)
 {
 }
 
-/* If the Guest expects to have an Advanced Programmable Interrupt Controller,
+/*
+ * If the Guest expects to have an Advanced Programmable Interrupt Controller,
  * we play dumb by ignoring writes and returning 0 for reads.  So it's no
  * longer Programmable nor Controlling anything, and I don't think 8 lines of
  * code qualifies for Advanced.  It will also never interrupt anything.  It
- * does, however, allow us to get through the Linux boot code. */
+ * does, however, allow us to get through the Linux boot code.
+ */
 #ifdef CONFIG_X86_LOCAL_APIC
 static void lguest_apic_write(u32 reg, u32 v)
 {
@@ -956,11 +1103,13 @@ static void lguest_safe_halt(void)
         kvm_hypercall0(LHCALL_HALT);
 }
 
-/* The SHUTDOWN hypercall takes a string to describe what's happening, and
+/*
+ * The SHUTDOWN hypercall takes a string to describe what's happening, and
  * an argument which says whether this to restart (reboot) the Guest or not.
  *
  * Note that the Host always prefers that the Guest speak in physical addresses
- * rather than virtual addresses, so we use __pa() here. */
+ * rather than virtual addresses, so we use __pa() here.
+ */
 static void lguest_power_off(void)
 {
         kvm_hypercall2(LHCALL_SHUTDOWN, __pa("Power down"),
@@ -991,8 +1140,10 @@ static __init char *lguest_memory_setup(void)
          * nice to move it back to lguest_init.  Patch welcome... */
         atomic_notifier_chain_register(&panic_notifier_list, &paniced);
 
-        /* The Linux bootloader header contains an "e820" memory map: the
-         * Launcher populated the first entry with our memory limit. */
+        /*
+         *The Linux bootloader header contains an "e820" memory map: the
+         * Launcher populated the first entry with our memory limit.
+         */
         e820_add_region(boot_params.e820_map[0].addr,
                           boot_params.e820_map[0].size,
                           boot_params.e820_map[0].type);
@@ -1001,16 +1152,17 @@ static __init char *lguest_memory_setup(void)
         return "LGUEST";
 }
 
-/* We will eventually use the virtio console device to produce console output,
+/*
+ * We will eventually use the virtio console device to produce console output,
  * but before that is set up we use LHCALL_NOTIFY on normal memory to produce
- * console output. */
+ * console output.
+ */
 static __init int early_put_chars(u32 vtermno, const char *buf, int count)
 {
         char scratch[17];
         unsigned int len = count;
 
-        /* We use a nul-terminated string, so we have to make a copy.  Icky,
-         * huh? */
+        /* We use a nul-terminated string, so we make a copy.  Icky, huh? */
         if (len > sizeof(scratch) - 1)
                 len = sizeof(scratch) - 1;
         scratch[len] = '\0';
@@ -1021,8 +1173,10 @@ static __init int early_put_chars(u32 vtermno, const char *buf, int count)
         return len;
 }
 
-/* Rebooting also tells the Host we're finished, but the RESTART flag tells the
- * Launcher to reboot us. */
+/*
+ * Rebooting also tells the Host we're finished, but the RESTART flag tells the
+ * Launcher to reboot us.
+ */
 static void lguest_restart(char *reason)
 {
         kvm_hypercall2(LHCALL_SHUTDOWN, __pa(reason), LGUEST_SHUTDOWN_RESTART);
@@ -1049,7 +1203,8 @@ static void lguest_restart(char *reason)
  * fit comfortably.
  *
  * First we need assembly templates of each of the patchable Guest operations,
- * and these are in i386_head.S. */
+ * and these are in i386_head.S.
+ */
 
 /*G:060 We construct a table from the assembler templates: */
 static const struct lguest_insns
@@ -1060,9 +1215,11 @@ static const struct lguest_insns
         [PARAVIRT_PATCH(pv_irq_ops.save_fl)] = { lgstart_pushf, lgend_pushf },
 };
 
-/* Now our patch routine is fairly simple (based on the native one in
+/*
+ * Now our patch routine is fairly simple (based on the native one in
  * paravirt.c).  If we have a replacement, we copy it in and return how much of
- * the available space we used. */
+ * the available space we used.
+ */
 static unsigned lguest_patch(u8 type, u16 clobber, void *ibuf,
                              unsigned long addr, unsigned len)
 {
@@ -1074,8 +1231,7 @@ static unsigned lguest_patch(u8 type, u16 clobber, void *ibuf,
 
         insn_len = lguest_insns[type].end - lguest_insns[type].start;
 
-        /* Similarly if we can't fit replacement (shouldn't happen, but let's
-         * be thorough). */
+        /* Similarly if it can't fit (doesn't happen, but let's be thorough). */
         if (len < insn_len)
                 return paravirt_patch_default(type, clobber, ibuf, addr, len);
 
@@ -1084,22 +1240,28 @@ static unsigned lguest_patch(u8 type, u16 clobber, void *ibuf,
         return insn_len;
 }
 
-/*G:029 Once we get to lguest_init(), we know we're a Guest.  The various
+/*G:029
+ * Once we get to lguest_init(), we know we're a Guest.  The various
  * pv_ops structures in the kernel provide points for (almost) every routine we
- * have to override to avoid privileged instructions. */
+ * have to override to avoid privileged instructions.
+ */
 __init void lguest_init(void)
 {
-        /* We're under lguest, paravirt is enabled, and we're running at
-         * privilege level 1, not 0 as normal. */
+        /* We're under lguest. */
         pv_info.name = "lguest";
+        /* Paravirt is enabled. */
         pv_info.paravirt_enabled = 1;
+        /* We're running at privilege level 1, not 0 as normal. */
         pv_info.kernel_rpl = 1;
+        /* Everyone except Xen runs with this set. */
         pv_info.shared_kernel_pmd = 1;
 
-        /* We set up all the lguest overrides for sensitive operations.  These
-         * are detailed with the operations themselves. */
+        /*
+         * We set up all the lguest overrides for sensitive operations.  These
+         * are detailed with the operations themselves.
+         */
 
-        /* interrupt-related operations */
+        /* Interrupt-related operations */
         pv_irq_ops.init_IRQ = lguest_init_IRQ;
         pv_irq_ops.save_fl = PV_CALLEE_SAVE(save_fl);
         pv_irq_ops.restore_fl = __PV_IS_CALLEE_SAVE(lg_restore_fl);
@@ -1107,11 +1269,11 @@ __init void lguest_init(void)
         pv_irq_ops.irq_enable = __PV_IS_CALLEE_SAVE(lg_irq_enable);
         pv_irq_ops.safe_halt = lguest_safe_halt;
 
-        /* init-time operations */
+        /* Setup operations */
         pv_init_ops.memory_setup = lguest_memory_setup;
         pv_init_ops.patch = lguest_patch;
 
-        /* Intercepts of various cpu instructions */
+        /* Intercepts of various CPU instructions */
         pv_cpu_ops.load_gdt = lguest_load_gdt;
         pv_cpu_ops.cpuid = lguest_cpuid;
         pv_cpu_ops.load_idt = lguest_load_idt;
@@ -1132,7 +1294,7 @@ __init void lguest_init(void)
         pv_cpu_ops.start_context_switch = paravirt_start_context_switch;
         pv_cpu_ops.end_context_switch = lguest_end_context_switch;
 
-        /* pagetable management */
+        /* Pagetable management */
         pv_mmu_ops.write_cr3 = lguest_write_cr3;
         pv_mmu_ops.flush_tlb_user = lguest_flush_tlb_user;
         pv_mmu_ops.flush_tlb_single = lguest_flush_tlb_single;
@@ -1154,54 +1316,71 @@ __init void lguest_init(void)
         pv_mmu_ops.pte_update_defer = lguest_pte_update;
 
 #ifdef CONFIG_X86_LOCAL_APIC
-        /* apic read/write intercepts */
+        /* APIC read/write intercepts */
         set_lguest_basic_apic_ops();
 #endif
 
-        /* time operations */
+        /* Time operations */
         pv_time_ops.get_wallclock = lguest_get_wallclock;
         pv_time_ops.time_init = lguest_time_init;
         pv_time_ops.get_tsc_khz = lguest_tsc_khz;
 
-        /* Now is a good time to look at the implementations of these functions
-         * before returning to the rest of lguest_init(). */
+        /*
+         * Now is a good time to look at the implementations of these functions
+         * before returning to the rest of lguest_init().
+         */
 
-        /*G:070 Now we've seen all the paravirt_ops, we return to
+        /*G:070
+         * Now we've seen all the paravirt_ops, we return to
          * lguest_init() where the rest of the fairly chaotic boot setup
-         * occurs. */
+         * occurs.
+         */
 
-        /* The stack protector is a weird thing where gcc places a canary
+        /*
+         * The stack protector is a weird thing where gcc places a canary
          * value on the stack and then checks it on return.  This file is
          * compiled with -fno-stack-protector it, so we got this far without
          * problems.  The value of the canary is kept at offset 20 from the
          * %gs register, so we need to set that up before calling C functions
-         * in other files. */
+         * in other files.
+         */
         setup_stack_canary_segment(0);
-        /* We could just call load_stack_canary_segment(), but we might as
-         * call switch_to_new_gdt() which loads the whole table and sets up
-         * the per-cpu segment descriptor register %fs as well. */
+
+        /*
+         * We could just call load_stack_canary_segment(), but we might as well
+         * call switch_to_new_gdt() which loads the whole table and sets up the
+         * per-cpu segment descriptor register %fs as well.
+         */
         switch_to_new_gdt(0);
 
-        /* As described in head_32.S, we map the first 128M of memory. */
+        /* We actually boot with all memory mapped, but let's say 128MB. */
         max_pfn_mapped = (128*1024*1024) >> PAGE_SHIFT;
 
-        /* The Host<->Guest Switcher lives at the top of our address space, and
+        /*
+         * The Host<->Guest Switcher lives at the top of our address space, and
          * the Host told us how big it is when we made LGUEST_INIT hypercall:
-         * it put the answer in lguest_data.reserve_mem  */
+         * it put the answer in lguest_data.reserve_mem
+         */
         reserve_top_address(lguest_data.reserve_mem);
 
-        /* If we don't initialize the lock dependency checker now, it crashes
-         * paravirt_disable_iospace. */
+        /*
+         * If we don't initialize the lock dependency checker now, it crashes
+         * paravirt_disable_iospace.
+         */
         lockdep_init();
 
-        /* The IDE code spends about 3 seconds probing for disks: if we reserve
+        /*
+         * The IDE code spends about 3 seconds probing for disks: if we reserve
          * all the I/O ports up front it can't get them and so doesn't probe.
          * Other device drivers are similar (but less severe).  This cuts the
-         * kernel boot time on my machine from 4.1 seconds to 0.45 seconds. */
+         * kernel boot time on my machine from 4.1 seconds to 0.45 seconds.
+         */
         paravirt_disable_iospace();
 
-        /* This is messy CPU setup stuff which the native boot code does before
-         * start_kernel, so we have to do, too: */
+        /*
+         * This is messy CPU setup stuff which the native boot code does before
+         * start_kernel, so we have to do, too:
+         */
         cpu_detect(&new_cpu_data);
         /* head.S usually sets up the first capability word, so do it here. */
         new_cpu_data.x86_capability[0] = cpuid_edx(1);
@@ -1218,22 +1397,28 @@ __init void lguest_init(void)
         acpi_ht = 0;
 #endif
 
-        /* We set the preferred console to "hvc".  This is the "hypervisor
+        /*
+         * We set the preferred console to "hvc".  This is the "hypervisor
          * virtual console" driver written by the PowerPC people, which we also
-         * adapted for lguest's use. */
+         * adapted for lguest's use.
+         */
         add_preferred_console("hvc", 0, NULL);
 
         /* Register our very early console. */
         virtio_cons_early_init(early_put_chars);
 
-        /* Last of all, we set the power management poweroff hook to point to
+        /*
+         * Last of all, we set the power management poweroff hook to point to
          * the Guest routine to power off, and the reboot hook to our restart
-         * routine. */
+         * routine.
+         */
         pm_power_off = lguest_power_off;
         machine_ops.restart = lguest_restart;
 
-        /* Now we're set up, call i386_start_kernel() in head32.c and we proceed
-         * to boot as normal.  It never returns. */
+        /*
+         * Now we're set up, call i386_start_kernel() in head32.c and we proceed
+         * to boot as normal.  It never returns.
+         */
         i386_start_kernel();
 }
 /*