Introduce "hcall" pointer to indicate pending hypercall.

Currently we look at the "trapnum" to see if the Guest wants a
hypercall.  But once the hypercall is done we have to reset trapnum to
a bogus value, otherwise if we exit to userspace and return, we'd run
the same hypercall twice (that was a nasty bug to find!).

This has two main effects:

1) When Jes's patch changes the hypercall args to be a generic "struct
   hcall_args" we simply change the type of "lg->hcall".  It's set by
   arch code, so if it has to copy args or something it can do so, and
   point "hcall" into lg->arch somewhere.

2) Async hypercalls only get run when an actual hypercall is pending.
   This simplfies the code a little and is a more logical semantic.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 20 insertions, 28 deletions

diff --git a/drivers/lguest/hypercalls.c b/drivers/lguest/hypercalls.c
index 8bde20934f91..0175a9f03347 100644
--- a/drivers/lguest/hypercalls.c
+++ b/drivers/lguest/hypercalls.c
@@ -241,19 +241,6 @@ static void initialize(struct lguest *lg)
  * is one other way we can do things for the Guest, as we see in
  * emulate_insn(). */
 
-/*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
  *
@@ -262,16 +249,12 @@ static void clear_hcall(struct lguest *lg)
  */
 void do_hypercalls(struct lguest *lg)
 {
-        /* Not initialized yet? */
+        /* Not initialized yet?  This hypercall must do it. */
         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);
-                }
+                /* Set up the "struct lguest_data" */
+                initialize(lg);
+                /* Hcall is done. */
+                lg->hcall = NULL;
                 return;
         }
 
@@ -281,12 +264,21 @@ void do_hypercalls(struct lguest *lg)
         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);
+         * SEND_DMA to the Launcher, we want to return now.  Otherwise we do
+         * the hypercall. */
+        if (!lg->dma_is_pending) {
+                do_hcall(lg, lg->hcall);
+                /* Tricky point: we reset the hcall pointer to mark the
+                 * hypercall as "done".  We use the hcall pointer rather than
+                 * the trap number to indicate a hypercall is pending.
+                 * Normally it doesn't matter: the Guest will run again and
+                 * update the trap number before we come back here.
+                 *
+                 * However, if we are signalled or the Guest sends DMA to the
+                 * Launcher, the run_guest() loop will exit without running the
+                 * Guest.  When it comes back it would try to re-run the
+                 * hypercall. */
+                lg->hcall = NULL;
         }
 }