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

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 03/31/2017 10:34 AM, Jan Beulich wrote:
>>>> 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.

I've managed to obtain this log of emulation errors:
https://pastebin.com/Esy1SkHx

The "virtual address" lines that are not followed by any "Mem event"
line correspond to CMXCHG_FAILED return codes.

The very last line is a MMIO emulation failed.

It's probably important that this happens with the model where
hvm_emulate_one_vm_event() does _not_ re-try the emulation until it
succeeds. The other model allows me to go further with the guest, but
eventually I get timeout-related BSODs or the guest becomes unresponsive.


Thanks,
Razvan

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