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

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 03/28/2017 01:47 PM, Jan Beulich wrote:
>>>> On 28.03.17 at 12:27, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>> On 03/28/2017 01:03 PM, Jan Beulich wrote:
>>>>>> On 28.03.17 at 11:14, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>>> I'm not sure that the RETRY model is what the guest OS expects. AFAIK, a
>>>> failed CMPXCHG should happen just once, with the proper registers and ZF
>>>> set. The guest surely expects neither that the instruction resume until
>>>> it succeeds, nor that some hidden loop goes on for an undeterminate
>>>> ammount of time until a CMPXCHG succeeds.
>>>
>>> The guest doesn't observe the CMPXCHG failing - RETRY leads to
>>> the instruction being restarted instead of completed.
>>
>> Indeed, but it works differently with hvm_emulate_one_vm_event() where
>> RETRY currently would have the instruction be re-executed (properly
>> re-executed, not just re-emulated) by the guest.
> 
> Right - see my other reply to Andrew: The function likely would
> need to tell apart guest CMPXCHG uses from us using the insn to
> carry out the write by some other one. That may involve
> adjustments to the memory write logic in x86_emulate() itself, as
> the late failure of the comparison then would also need to be
> communicated back (via ZF clear) to the guest.

Exactly, it would require quite some reworking of x86_emulate().


Thanks,
Razvan

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