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>
diff --git a/drivers/lguest/hypercalls.c b/drivers/lguest/hypercalls.c
index fb546b0..7a5299f 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 @@
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 @@
}
}
-/* 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 @@
break;
}
+ /* Do the hypercall, same as a normal one. */
do_hcall(lg, ®s);
+
+ /* 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);
}
}