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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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