lguest: documentation V: Host

Documentation: The Host

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 99 insertions, 10 deletions

diff --git a/drivers/lguest/segments.c b/drivers/lguest/segments.c
index c4fc7293b84b..4d4e5a4586f9 100644
--- a/drivers/lguest/segments.c
+++ b/drivers/lguest/segments.c
@@ -11,17 +11,58 @@
  * from frolicking through its parklike serenity. :*/
 #include "lg.h"
 
+/*H:600
+ * We've almost completed the Host; there's just one file to go!
+ *
+ * Segments & The Global Descriptor Table
+ *
+ * (That title sounds like a bad Nerdcore group.  Not to suggest that there are
+ * any good Nerdcore groups, but in high school a friend of mine had a band
+ * called Joe Fish and the Chips, so there are definitely worse band names).
+ *
+ * To refresh: the GDT is a table of 8-byte values describing segments.  Once
+ * set up, these segments can be loaded into one of the 6 "segment registers".
+ *
+ * GDT entries are passed around as "struct desc_struct"s, which like IDT
+ * entries are split into two 32-bit members, "a" and "b".  One day, someone
+ * will clean that up, and be declared a Hero.  (No pressure, I'm just saying).
+ *
+ * Anyway, the GDT entry contains a base (the start address of the segment), a
+ * limit (the size of the segment - 1), and some flags.  Sounds simple, and it
+ * would be, except those zany Intel engineers decided that it was too boring
+ * to put the base at one end, the limit at the other, and the flags in
+ * between.  They decided to shotgun the bits at random throughout the 8 bytes,
+ * like so:
+ *
+ * 0               16                     40       48  52  56     63
+ * [ limit part 1 ][     base part 1     ][ flags ][li][fl][base ]
+ *                                                  mit ags part 2
+ *                                                part 2
+ *
+ * As a result, this file contains a certain amount of magic numeracy.  Let's
+ * begin.
+ */
+
+/* Is the descriptor the Guest wants us to put in OK?
+ *
+ * The flag which Intel says must be zero: must be zero.  The descriptor must
+ * be present, (this is actually checked earlier but is here for thorougness),
+ * and the descriptor type must be 1 (a memory segment).  */
 static int desc_ok(const struct desc_struct *gdt)
 {
-        /* MBZ=0, P=1, DT=1  */
         return ((gdt->b & 0x00209000) == 0x00009000);
 }
 
+/* Is the segment present?  (Otherwise it can't be used by the Guest). */
 static int segment_present(const struct desc_struct *gdt)
 {
         return gdt->b & 0x8000;
 }
 
+/* There are several entries we don't let the Guest set.  The TSS entry is the
+ * "Task State Segment" which controls all kinds of delicate things.  The
+ * LGUEST_CS and LGUEST_DS entries are reserved for the Switcher, and the
+ * the Guest can't be trusted to deal with double faults. */
 static int ignored_gdt(unsigned int num)
 {
         return (num == GDT_ENTRY_TSS
@@ -30,9 +71,18 @@ static int ignored_gdt(unsigned int num)
                 || num == GDT_ENTRY_DOUBLEFAULT_TSS);
 }
 
-/* We don't allow removal of CS, DS or SS; it doesn't make sense. */
+/* If the Guest asks us to remove an entry from the GDT, we have to be careful.
+ * If one of the segment registers is pointing at that entry the Switcher will
+ * crash when it tries to reload the segment registers for the Guest.
+ *
+ * It doesn't make much sense for the Guest to try to remove its own code, data
+ * or stack segments while they're in use: assume that's a Guest bug.  If it's
+ * one of the lesser segment registers using the removed entry, we simply set
+ * that register to 0 (unusable). */
 static void check_segment_use(struct lguest *lg, unsigned int desc)
 {
+        /* GDT entries are 8 bytes long, so we divide to get the index and
+         * ignore the bottom bits. */
         if (lg->regs->gs / 8 == desc)
                 lg->regs->gs = 0;
         if (lg->regs->fs / 8 == desc)
@@ -45,12 +95,16 @@ static void check_segment_use(struct lguest *lg, unsigned int desc)
                 kill_guest(lg, "Removed live GDT entry %u", desc);
 }
 
+/*H:610 Once the GDT has been changed, we look through the changed entries and
+ * see if they're OK.  If not, we'll call kill_guest() and the Guest will never
+ * get to use the invalid entries. */
 static void fixup_gdt_table(struct lguest *lg, unsigned start, unsigned end)
 {
         unsigned int i;
 
         for (i = start; i < end; i++) {
-                /* We never copy these ones to real gdt */
+                /* We never copy these ones to real GDT, so we don't care what
+                 * they say */
                 if (ignored_gdt(i))
                         continue;
 
@@ -64,41 +118,57 @@ static void fixup_gdt_table(struct lguest *lg, unsigned start, unsigned end)
                 if (!desc_ok(&lg->gdt[i]))
                         kill_guest(lg, "Bad GDT descriptor %i", i);
 
-                /* DPL 0 presumably means "for use by guest". */
+                /* Segment descriptors contain a privilege level: the Guest is
+                 * sometimes careless and leaves this as 0, even though it's
+                 * running at privilege level 1.  If so, we fix it here. */
                 if ((lg->gdt[i].b & 0x00006000) == 0)
                         lg->gdt[i].b |= (GUEST_PL << 13);
 
-                /* Set accessed bit, since gdt isn't writable. */
+                /* Each descriptor has an "accessed" bit.  If we don't set it
+                 * now, the CPU will try to set it when the Guest first loads
+                 * that entry into a segment register.  But the GDT isn't
+                 * writable by the Guest, so bad things can happen. */
                 lg->gdt[i].b |= 0x00000100;
         }
 }
 
+/* This routine is called at boot or modprobe time for each CPU to set up the
+ * "constant" GDT entries for Guests running on that CPU. */
 void setup_default_gdt_entries(struct lguest_ro_state *state)
 {
         struct desc_struct *gdt = state->guest_gdt;
         unsigned long tss = (unsigned long)&state->guest_tss;
 
-        /* Hypervisor segments. */
+        /* The hypervisor segments are full 0-4G segments, privilege level 0 */
         gdt[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT;
         gdt[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT;
 
-        /* This is the one which we *cannot* copy from guest, since tss
-           is depended on this lguest_ro_state, ie. this cpu. */
+        /* The TSS segment refers to the TSS entry for this CPU, so we cannot
+         * copy it from the Guest.  Forgive the magic flags */
         gdt[GDT_ENTRY_TSS].a = 0x00000067 | (tss << 16);
         gdt[GDT_ENTRY_TSS].b = 0x00008900 | (tss & 0xFF000000)
                 | ((tss >> 16) & 0x000000FF);
 }
 
+/* This routine is called before the Guest is run for the first time. */
 void setup_guest_gdt(struct lguest *lg)
 {
+        /* Start with full 0-4G segments... */
         lg->gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT;
         lg->gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT;
+        /* ...except the Guest is allowed to use them, so set the privilege
+         * level appropriately in the flags. */
         lg->gdt[GDT_ENTRY_KERNEL_CS].b |= (GUEST_PL << 13);
         lg->gdt[GDT_ENTRY_KERNEL_DS].b |= (GUEST_PL << 13);
 }
 
-/* This is a fast version for the common case where only the three TLS entries
- * have changed. */
+/* Like the IDT, we never simply use the GDT the Guest gives us.  We set up the
+ * GDTs for each CPU, then we copy across the entries each time we want to run
+ * a different Guest on that CPU. */
+
+/* A partial GDT load, for the three "thead-local storage" entries.  Otherwise
+ * it's just like load_guest_gdt().  So much, in fact, it would probably be
+ * neater to have a single hypercall to cover both. */
 void copy_gdt_tls(const struct lguest *lg, struct desc_struct *gdt)
 {
         unsigned int i;
@@ -107,22 +177,31 @@ void copy_gdt_tls(const struct lguest *lg, struct desc_struct *gdt)
                 gdt[i] = lg->gdt[i];
 }
 
+/* This is the full version */
 void copy_gdt(const struct lguest *lg, struct desc_struct *gdt)
 {
         unsigned int i;
 
+        /* The default entries from setup_default_gdt_entries() are not
+         * replaced.  See ignored_gdt() above. */
         for (i = 0; i < GDT_ENTRIES; i++)
                 if (!ignored_gdt(i))
                         gdt[i] = lg->gdt[i];
 }
 
+/* This is where the Guest asks us to load a new GDT (LHCALL_LOAD_GDT). */
 void load_guest_gdt(struct lguest *lg, unsigned long table, u32 num)
 {
+        /* We assume the Guest has the same number of GDT entries as the
+         * Host, otherwise we'd have to dynamically allocate the Guest GDT. */
         if (num > ARRAY_SIZE(lg->gdt))
                 kill_guest(lg, "too many gdt entries %i", num);
 
+        /* We read the whole thing in, then fix it up. */
         lgread(lg, lg->gdt, table, num * sizeof(lg->gdt[0]));
         fixup_gdt_table(lg, 0, ARRAY_SIZE(lg->gdt));
+        /* Mark that the GDT changed so the core knows it has to copy it again,
+         * even if the Guest is run on the same CPU. */
         lg->changed |= CHANGED_GDT;
 }
 
@@ -134,3 +213,13 @@ void guest_load_tls(struct lguest *lg, unsigned long gtls)
         fixup_gdt_table(lg, GDT_ENTRY_TLS_MIN, GDT_ENTRY_TLS_MAX+1);
         lg->changed |= CHANGED_GDT_TLS;
 }
+
+/*
+ * With this, we have finished the Host.
+ *
+ * Five of the seven parts of our task are complete.  You have made it through
+ * the Bit of Despair (I think that's somewhere in the page table code,
+ * myself).
+ *
+ * Next, we examine "make Switcher".  It's short, but intense.
+ */