Re: [Xen-devel] Future x86 emulator direction

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 12/14/2016 09:37 AM, Razvan Cojocaru wrote:
> On 12/14/2016 09:14 AM, Jan Beulich wrote:
>>>>> On 13.12.16 at 23:02, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> On 13/12/2016 21:55, Razvan Cojocaru wrote:
>>>> On a somewhat related note, it's important to also figure out how best
>>>> to avoid emulation races such as the LOCK CMPXCHG issue we've discussed
>>>> in the past. Maybe that's also worth taking into consideration at this
>>>> early stage.
>>>
>>> Funny you should ask that.
>>>
>>> The only possible way to do this safely is to have the emulator map the
>>> target frame(s) and execute a locked stub instruction with a memory
>>> operand pointing at the mapping.  We have no other way of interacting
>>> with the cache coherency fabric.
>>
>> Well, that approach is necessary only if one path (vCPU) can write
>> to a page, while another one needs emulation. If pages are globally
>> write-protected, an approach following the model from Razvan's
>> earlier patch (which I have no idea what has become of) would
>> seem to suffice.
> 
> As previously stated, you've raised performance concerns which seemed to
> require a different direction, namely the one Andrew is now suggesting,
> which indeed, aside from being somewhat faster is also safer for all
> cases (including the one you've mentioned, where one path can write
> normally and the other does so via emulation).
> 
> The old patch itself is still alive in the XenServer patch queue, albeit
> quite unlikely to be trivial to apply to the current Xen 4.9-unstable
> code in its current form:
> 
> https://github.com/xenserver/xen-4.7.pg/blob/master/master/xen-x86-emulate-syncrhonise-LOCKed-instruction-emulation.patch
> 
> Again, if you decide that this patch is preferable, I can try to rework
> it for the current version of Xen.

Sorry to revive this old thread, but I'm still not sure what the
upstream solution for this very real problem should be. Should I bring
back the old patch that synchronizes LOCKed CMPXCHGs (perhaps with
Andrew's kind help, as he's stated that they keep an up-to-date patch
that works against staging)? Or are you considering implementing a stub
as part of the work being done on the emulator?


Thanks,
Razvan

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