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, 108 insertions, 10 deletions

diff --git a/drivers/lguest/hypercalls.c b/drivers/lguest/hypercalls.c
index fb546b046445..7a5299f9679d 100644
--- a/drivers/lguest/hypercalls.c
+++ b/drivers/lguest/hypercalls.c
@@ -28,37 +28,63 @@
 #include <irq_vectors.h>
 #include "lg.h"
 
+/*H:120 This is the core hypercall routine: where the Guest gets what it
+ * wants.  Or gets killed.  Or, in the case of LHCALL_CRASH, both.
+ *
+ * Remember from the Guest: %eax == which call to make, and the arguments are
+ * packed into %edx, %ebx and %ecx if needed. */
 static void do_hcall(struct lguest *lg, struct lguest_regs *regs)
 {
         switch (regs->eax) {
         case LHCALL_FLUSH_ASYNC:
+                /* This call does nothing, except by breaking out of the Guest
+                 * it makes us process all the asynchronous hypercalls. */
                 break;
         case LHCALL_LGUEST_INIT:
+                /* You can't get here unless you're already initialized.  Don't
+                 * do that. */
                 kill_guest(lg, "already have lguest_data");
                 break;
         case LHCALL_CRASH: {
+                /* Crash is such a trivial hypercall that we do it in four
+                 * lines right here. */
                 char msg[128];
+                /* If the lgread fails, it will call kill_guest() itself; the
+                 * kill_guest() with the message will be ignored. */
                 lgread(lg, msg, regs->edx, sizeof(msg));
                 msg[sizeof(msg)-1] = '\0';
                 kill_guest(lg, "CRASH: %s", msg);
                 break;
         }
         case LHCALL_FLUSH_TLB:
+                /* FLUSH_TLB comes in two flavors, depending on the
+                 * argument: */
                 if (regs->edx)
                         guest_pagetable_clear_all(lg);
                 else
                         guest_pagetable_flush_user(lg);
                 break;
         case LHCALL_GET_WALLCLOCK: {
+                /* The Guest wants to know the real time in seconds since 1970,
+                 * in good Unix tradition. */
                 struct timespec ts;
                 ktime_get_real_ts(&ts);
                 regs->eax = ts.tv_sec;
                 break;
         }
         case LHCALL_BIND_DMA:
+                /* BIND_DMA really wants four arguments, but it's the only call
+                 * which does.  So the Guest packs the number of buffers and
+                 * the interrupt number into the final argument, and we decode
+                 * it here.  This can legitimately fail, since we currently
+                 * place a limit on the number of DMA pools a Guest can have.
+                 * So we return true or false from this call. */
                 regs->eax = bind_dma(lg, regs->edx, regs->ebx,
                                      regs->ecx >> 8, regs->ecx & 0xFF);
                 break;
+
+        /* All these calls simply pass the arguments through to the right
+         * routines. */
         case LHCALL_SEND_DMA:
                 send_dma(lg, regs->edx, regs->ebx);
                 break;
@@ -86,10 +112,13 @@ static void do_hcall(struct lguest *lg, struct lguest_regs *regs)
         case LHCALL_SET_CLOCKEVENT:
                 guest_set_clockevent(lg, regs->edx);
                 break;
+
         case LHCALL_TS:
+                /* This sets the TS flag, as we saw used in run_guest(). */
                 lg->ts = regs->edx;
                 break;
         case LHCALL_HALT:
+                /* Similarly, this sets the halted flag for run_guest(). */
                 lg->halted = 1;
                 break;
         default:
@@ -97,25 +126,42 @@ static void do_hcall(struct lguest *lg, struct lguest_regs *regs)
         }
 }
 
-/* We always do queued calls before actual hypercall. */
+/* Asynchronous hypercalls are easy: we just look in the array in the Guest's
+ * "struct lguest_data" and see if there are any new ones marked "ready".
+ *
+ * We are careful to do these in order: obviously we respect the order the
+ * Guest put them in the ring, but we also promise the Guest that they will
+ * happen before any normal hypercall (which is why we check this before
+ * checking for a normal hcall). */
 static void do_async_hcalls(struct lguest *lg)
 {
         unsigned int i;
         u8 st[LHCALL_RING_SIZE];
 
+        /* For simplicity, we copy the entire call status array in at once. */
         if (copy_from_user(&st, &lg->lguest_data->hcall_status, sizeof(st)))
                 return;
 
+
+        /* We process "struct lguest_data"s hcalls[] ring once. */
         for (i = 0; i < ARRAY_SIZE(st); i++) {
                 struct lguest_regs regs;
+                /* We remember where we were up to from last time.  This makes
+                 * sure that the hypercalls are done in the order the Guest
+                 * places them in the ring. */
                 unsigned int n = lg->next_hcall;
 
+                /* 0xFF means there's no call here (yet). */
                 if (st[n] == 0xFF)
                         break;
 
+                /* OK, we have hypercall.  Increment the "next_hcall" cursor,
+                 * and wrap back to 0 if we reach the end. */
                 if (++lg->next_hcall == LHCALL_RING_SIZE)
                         lg->next_hcall = 0;
 
+                /* We copy the hypercall arguments into a fake register
+                 * structure.  This makes life simple for do_hcall(). */
                 if (get_user(regs.eax, &lg->lguest_data->hcalls[n].eax)
                     || get_user(regs.edx, &lg->lguest_data->hcalls[n].edx)
                     || get_user(regs.ecx, &lg->lguest_data->hcalls[n].ecx)
@@ -124,74 +170,126 @@ static void do_async_hcalls(struct lguest *lg)
                         break;
                 }
 
+                /* Do the hypercall, same as a normal one. */
                 do_hcall(lg, &regs);
+
+                /* Mark the hypercall done. */
                 if (put_user(0xFF, &lg->lguest_data->hcall_status[n])) {
                         kill_guest(lg, "Writing result for async hypercall");
                         break;
                 }
 
+                /* Stop doing hypercalls if we've just done a DMA to the
+                 * Launcher: it needs to service this first. */
                 if (lg->dma_is_pending)
                         break;
         }
 }
 
+/* Last of all, we look at what happens first of all.  The very first time the
+ * Guest makes a hypercall, we end up here to set things up: */
 static void initialize(struct lguest *lg)
 {
         u32 tsc_speed;
 
+        /* You can't do anything until you're initialized.  The Guest knows the
+         * rules, so we're unforgiving here. */
         if (lg->regs->eax != LHCALL_LGUEST_INIT) {
                 kill_guest(lg, "hypercall %li before LGUEST_INIT",
                            lg->regs->eax);
                 return;
         }
 
-        /* We only tell the guest to use the TSC if it's reliable. */
+        /* We insist that the Time Stamp Counter exist and doesn't change with
+         * cpu frequency.  Some devious chip manufacturers decided that TSC
+         * changes could be handled in software.  I decided that time going
+         * backwards might be good for benchmarks, but it's bad for users.
+         *
+         * We also insist that the TSC be stable: the kernel detects unreliable
+         * TSCs for its own purposes, and we use that here. */
         if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && !check_tsc_unstable())
                 tsc_speed = tsc_khz;
         else
                 tsc_speed = 0;
 
+        /* The pointer to the Guest's "struct lguest_data" is the only
+         * argument. */
         lg->lguest_data = (struct lguest_data __user *)lg->regs->edx;
-        /* We check here so we can simply copy_to_user/from_user */
+        /* If we check the address they gave is OK now, we can simply
+         * copy_to_user/from_user from now on rather than using lgread/lgwrite.
+         * I put this in to show that I'm not immune to writing stupid
+         * optimizations. */
         if (!lguest_address_ok(lg, lg->regs->edx, sizeof(*lg->lguest_data))) {
                 kill_guest(lg, "bad guest page %p", lg->lguest_data);
                 return;
         }
+        /* The Guest tells us where we're not to deliver interrupts by putting
+         * the range of addresses into "struct lguest_data". */
         if (get_user(lg->noirq_start, &lg->lguest_data->noirq_start)
             || get_user(lg->noirq_end, &lg->lguest_data->noirq_end)
-            /* We reserve the top pgd entry. */
+            /* We tell the Guest that it can't use the top 4MB of virtual
+             * addresses used by the Switcher. */
             || put_user(4U*1024*1024, &lg->lguest_data->reserve_mem)
             || put_user(tsc_speed, &lg->lguest_data->tsc_khz)
+            /* We also give the Guest a unique id, as used in lguest_net.c. */
             || put_user(lg->guestid, &lg->lguest_data->guestid))
                 kill_guest(lg, "bad guest page %p", lg->lguest_data);
 
-        /* This is the one case where the above accesses might have
-         * been the first write to a Guest page.  This may have caused
-         * a copy-on-write fault, but the Guest might be referring to
-         * the old (read-only) page. */
+        /* This is the one case where the above accesses might have been the
+         * first write to a Guest page.  This may have caused a copy-on-write
+         * fault, but the Guest might be referring to the old (read-only)
+         * page. */
         guest_pagetable_clear_all(lg);
 }
+/* Now we've examined the hypercall code; our Guest can make requests.  There
+ * is one other way we can do things for the Guest, as we see in
+ * emulate_insn(). */
 
-/* Even if we go out to userspace and come back, we don't want to do
- * the hypercall again. */
+/*H:110 Tricky point: we mark the hypercall as "done" once we've done it.
+ * Normally we don't need to do this: the Guest will run again and update the
+ * trap number before we come back around the run_guest() loop to
+ * do_hypercalls().
+ *
+ * However, if we are signalled or the Guest sends DMA to the Launcher, that
+ * loop will exit without running the Guest.  When it comes back it would try
+ * to re-run the hypercall. */
 static void clear_hcall(struct lguest *lg)
 {
         lg->regs->trapnum = 255;
 }
 
+/*H:100
+ * Hypercalls
+ *
+ * Remember from the Guest, hypercalls come in two flavors: normal and
+ * asynchronous.  This file handles both of types.
+ */
 void do_hypercalls(struct lguest *lg)
 {
+        /* Not initialized yet? */
         if (unlikely(!lg->lguest_data)) {
+                /* Did the Guest make a hypercall?  We might have come back for
+                 * some other reason (an interrupt, a different trap). */
                 if (lg->regs->trapnum == LGUEST_TRAP_ENTRY) {
+                        /* Set up the "struct lguest_data" */
                         initialize(lg);
+                        /* The hypercall is done. */
                         clear_hcall(lg);
                 }
                 return;
         }
 
+        /* The Guest has initialized.
+         *
+         * Look in the hypercall ring for the async hypercalls: */
         do_async_hcalls(lg);
+
+        /* If we stopped reading the hypercall ring because the Guest did a
+         * SEND_DMA to the Launcher, we want to return now.  Otherwise if the
+         * Guest asked us to do a hypercall, we do it. */
         if (!lg->dma_is_pending && lg->regs->trapnum == LGUEST_TRAP_ENTRY) {
                 do_hcall(lg, lg->regs);
+                /* The hypercall is done. */
                 clear_hcall(lg);
         }
 }