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Re: [Xen-devel] [for-4.7] x86/emulate: synchronize LOCKed instruction emulation

On 04/27/2016 09:22 AM, Jan Beulich wrote:
>>>> On 26.04.16 at 19:23, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>> On 04/26/16 19:03, George Dunlap wrote:
>>> On 19/04/16 17:35, Jan Beulich wrote:
>>>>>>> Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx> 04/19/16 1:01 PM >>>
>>>>> I think this might be because the LOCK prefix should guarantee that the
>>>>> instruction that follows it has exclusive use of shared memory (for both
>>>>> reads and writes) but I might be misreading the docs:
>>>> LOCK definitely has no effect on other than the instruction it gets applied
>>>> to.
>>> Sorry I wasn't involved in this discussion -- what was the conclusion here?
>>> FWIW Andy's suggestion of using a stub seemed like the most robust
>>> solution, if that could be made to work.
>>> If you're going to submit a patch substantially similar to this one, let
>>> me know so I can review the mm bits of the original patch.
>> I'm not really sure.
>> Regarding this version of the patch, Jan has asked for more information
>> on the performance impact, but I'm not sure how to obtain it in a
>> rigorous manner.
> That was only one half, which Andrew has now answered. The
> other was the not understood issue with a later variant you had
> tried.

Right, there's the fact that this version (with read / write locking the
whole function) works whereas just synchonizing hvmemul_cmpxchg() with a
spin lock does not. It is not yet fully clear why (the part of the
conversation at the very top of this email was an early attempt to
elucidate it).


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