Re: [Xen-devel] [PATCH RFC] x86/emulate: implement hvmemul_cmpxchg() with an actual CMPXCHG

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 31.03.17 at 08:17, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> On 03/30/2017 06:47 PM, Jan Beulich wrote:
>>> Speaking of emulated MMIO, I've got this when the guest was crashing
>>> immediately (pre RETRY loop):
>>>
>>>  MMIO emulation failed: d3v8 32bit @ 0008:82679f3c -> f0 0f ba 30 00 72
>>> 07 8b cb e8 da 4b ff ff 8b 45
>> 
>> That's a BTR, which we should be emulating fine. More information
>> would need to be collected to have a chance to understand what
>> might be going one (first of all the virtual and physical memory
>> address this was trying to act on).
> 
> Right, the BTR part should be fine, but I think the LOCK part is what's
> causing the issue. I've done a few more test runs to see what return
> RETRY (dumping the instruction with an "(r)" prefix to distinguish from
> the UNHANDLEABLE dump), and a couple of instructions return RETRY (BTR
> and XADD, both LOCK-prefixed, which means they now involve CMPXCHG
> handler, which presumably now fails - possibly simply because it's
> always LOCKed in my patch):

Well, all of that looks to be expected behavior. I'm afraid I don't see
how this information helps understanding the MMIO emulation failure
above.

Jan


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