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);
 	}
 }