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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.03.17 at 12:50, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> 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().

I had imagined it to be less intrusive (outside of x86_emulate()),
but I've now learned why Andrew was able to get rid of
X86EMUL_CMPXCHG_FAILED - the apparently intended behavior
was never implemented. Attached a first take at it, which has
seen smoke testing, but nothing more. The way it ends up being
I don't think this can reasonably be considered for 4.9 at this
point in time. (Also Cc-ing Tim for the shadow code changes,
even if this isn't really a proper patch submission.)

Jan

Attachment: x86emul-cmpxchg-fail.patch
Description: Text document

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