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Re: [Xen-devel] Possible problem emulating movntq, movss

>>> On 06.08.14 at 12:47, <vlutas@xxxxxxxxxxxxxxx> wrote:
> On 8/6/2014 12:54 PM, Jan Beulich wrote:
>>>>> On 06.08.14 at 10:57, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>> We found that our HVM guests froze when trying to emulate movntq
>>> instructions. The solution seems to be to replace "goto done;" with
>>> "break;" at line 4191 (when handling "case 0x7f:") in
>>> xen/arch/x86/x86_emulate/x86_emulate.c. Otherwise the writeback part
>>> doesn't happen.
>>> If you're happy with the fix I can prepare a patch, otherwise please let
>>> me know if we're missing something.
>> No, that doesn't look right: There's nothing left to be written back at
>> that point (registers get updated with the instruction executed via the
>> on-stack stub, and memory gets written with immediately preceding
>> ops->write(). So without you being more specific about _what_ you
>> see going wrong I don't think I can give further advice.
> Except for maybe the instruction pointer? That doesn't seem to be updated
> anywhereexcept during the write-back phase (or maybe I'm missing the spot).
> The problem is that the guest gets stuck with the instruction pointer
> pointing to the sameinstruction (in our particular case it is
> "MOVDQU xmm0, xmmword ptr [rdx + rcx - 0x10]"),entering in an infinite
> loop (EPT violation - emulate), since the IP doesn't seem to be updated.

Now that is indeed a problem, but not solved by simply replacing
the "goto done" with "break". I'll look into getting you a proper fix.


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