Re: [Xen-devel] [PATCH V2] x86/emulate: synchronize LOCKed instruction emulation

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.03.17 at 16:38, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> On 03/15/2017 06:57 PM, Jan Beulich wrote:
>>>>> On 15.03.17 at 17:46, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>> On 03/15/2017 06:30 PM, Jan Beulich wrote:
>>>>>>> On 15.03.17 at 17:04, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>>>> ---
>>>>> Changes since V1:
>>>>>  - Added Andrew Cooper's credit, as he's kept the patch current
>>>>>    througout non-trivial code changes since the initial patch.
>>>>>  - Significantly more patch testing (with XenServer).
>>>>>  - Restricted lock scope.
>>>>
>>>> Not by much, as it seems. In particular you continue to take the
>>>> lock even for instructions not accessing memory at all.
>>>
>>> I'll take a closer look.
>>>
>>>> Also, by "reworked" I did assume you mean converted to at least the
>>>> cmpxchg based model.
>>>
>>> I haven't been able to follow the latest emulator changes closely, could
>>> you please clarify what you mean by "the cmpxchg model"? Thanks.
>> 
>> This is unrelated to any recent changes. The idea is to make the
>> ->cmpxchg() hook actually behave like what its name says. It's
>> being used for LOCKed insn writeback already, and it could
>> therefore simply force a retry of the full instruction if the compare
>> part of it fails. It may need to be given another parameter, to
>> allow the hook function to tell LOCKed from "normal" uses.
> 
> I assume this is what you have in mind?

Hmm, not exactly. I'd expect an actual CMPXCHG instruction to
be used, with a LOCK prefix when the original access had one.
There should certainly not be any tail call to hvmemul_write()
anymore.

Jan


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