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Re: [Xen-devel] [PATCH 2/2] Xen/mem_event: Prevent underflow of vcpu pause counts

On Thu, 2014-07-17 at 14:38 -0400, Andres Lagar Cavilla wrote:
>         +    BUG_ON(test_and_set_bool(v->paused_for_mem_event) != 0);
> This is a problem. It relies on a vcpu being able to cause a single
> mem event during a vmexit. I don't think that can be guaranteed. While
> I can't pinpoint the exact conversation from years ago, it is not hard
> to imagine scenarios in which an mmio emulation can touch multiple
> pages.

Since some x86 instructions can have both its input and output in memory
it's not hard to imagine both being MMIO, at least if you were trying to
break things...


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