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[Xen-devel] Interrupt issues with hvm_emulate_one_vm_event()


I've noticed that, with pages marked NX and vm_event emulation, we can
end up emulating an ud2, for which hvm_emulate_one() returns
X86EMUL_EXCEPTION in hvm_emulate_one_vm_event().

This, in turn, causes a hvm_inject_event() call in the context of
hvm_do_resume(), which can, if there's already a pending event there,
cause a 101 BSOD (timer-related, if I understand correctly) or loss of
input (mouse frozen, keyboard unresponsive).

After much trial and error, I've been able to confirm this by leaving a
guest on for almost a full day with this change:

     case X86EMUL_EXCEPTION:
-        hvm_inject_event(&ctx.ctxt.event);
+        if ( !hvm_event_pending(current) )
+            hvm_inject_event(&ctx.ctxt.event);

and checking that there's been no BSOD or loss of input.

However, just losing the event here, while fine to prove that this is
indeed the problem, is not OK. But I'm not sure what an elegant / robust
way of fixing this is.

Suggestions are (as always) greatly appreciated.


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