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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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...

Ian.



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