[Xen-devel] [PATCH V3] mem_event: Allow emulating an instruction that caused a page fault

[Razvan Cojocaru <rzvncj@xxxxxxxxx>]

This patch makes it possible to emulate an instruction that triggered
a page fault (received via the mem_event API). This is done by setting
the MEM_EVENT_FLAG_EMULATE in mem_event_response_t.flags. The purpose
of this is to be able to receive several distinct page fault mem_events
for the same address, and choose which ones are allowed to go through
from dom0 userspace.

Signed-off-by: Razvan Cojocaru <rzvncj@xxxxxxxxx>

diff -r b6195e277da5 -r 8c614906bb81 xen/arch/x86/mm/p2m.c
--- a/xen/arch/x86/mm/p2m.c     Wed Jan 16 14:15:44 2013 +0000
+++ b/xen/arch/x86/mm/p2m.c     Thu Jan 17 18:26:24 2013 +0200
@@ -1309,6 +1309,17 @@ bool_t p2m_mem_access_check(paddr_t gpa,
         }
     }
 
+    if ( v->arch.mem_event_emulate )
+    {
+        struct hvm_emulate_ctxt ctx[1] = {};
+
+        v->arch.mem_event_emulate = 0;
+        hvm_emulate_prepare(ctx, guest_cpu_user_regs());
+        hvm_emulate_one(ctx);
+
+        return 1;
+    }
+
     *req_ptr = NULL;
     req = xzalloc(mem_event_request_t);
     if ( req )
@@ -1347,8 +1358,15 @@ void p2m_mem_access_resume(struct domain
     /* Pull all responses off the ring */
     while( mem_event_get_response(d, &d->mem_event->access, &rsp) )
     {
+        d->vcpu[rsp.vcpu_id]->arch.mem_event_emulate = 0;
+
         if ( rsp.flags & MEM_EVENT_FLAG_DUMMY )
             continue;
+
+        /* Mark vcpu for skipping one instruction upon rescheduling */
+        if ( rsp.flags & MEM_EVENT_FLAG_EMULATE )
+            d->vcpu[rsp.vcpu_id]->arch.mem_event_emulate = 1;
+
         /* Unpause domain */
         if ( rsp.flags & MEM_EVENT_FLAG_VCPU_PAUSED )
             vcpu_unpause(d->vcpu[rsp.vcpu_id]);
diff -r b6195e277da5 -r 8c614906bb81 xen/include/asm-x86/domain.h
--- a/xen/include/asm-x86/domain.h      Wed Jan 16 14:15:44 2013 +0000
+++ b/xen/include/asm-x86/domain.h      Thu Jan 17 18:26:24 2013 +0200
@@ -446,6 +446,8 @@ struct arch_vcpu
 
     void *compat_arg_xlat;
 
+    /* Should we emulate the first instruction on VCPU resume after a 
mem_event? */
+    uint8_t              mem_event_emulate;
 } __cacheline_aligned;
 
 /* Shorthands to improve code legibility. */
diff -r b6195e277da5 -r 8c614906bb81 xen/include/public/mem_event.h
--- a/xen/include/public/mem_event.h    Wed Jan 16 14:15:44 2013 +0000
+++ b/xen/include/public/mem_event.h    Thu Jan 17 18:26:24 2013 +0200
@@ -36,6 +36,8 @@
 #define MEM_EVENT_FLAG_EVICT_FAIL   (1 << 2)
 #define MEM_EVENT_FLAG_FOREIGN      (1 << 3)
 #define MEM_EVENT_FLAG_DUMMY        (1 << 4)
+/* Emulate the instruction that caused the current mem_event */
+#define MEM_EVENT_FLAG_EMULATE      (1 << 5)
 
 /* Reasons for the memory event request */
 #define MEM_EVENT_REASON_UNKNOWN     0    /* typical reason */

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