Re: [Xen-devel] [PATCH V2 1/2] xen, libxc: Fine grained control of REP emulation optimizations

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.09.15 at 17:28, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> On 09/22/2015 06:17 PM, Jan Beulich wrote:
>>>>> On 21.09.15 at 15:31, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>> --- a/xen/arch/x86/hvm/emulate.c
>>> +++ b/xen/arch/x86/hvm/emulate.c
>>> @@ -514,7 +514,8 @@ static int hvmemul_virtual_to_linear(
>>>       * being triggered for repeated writes to a whole page.
>>>       */
>>>      *reps = min_t(unsigned long, *reps,
>>> -                  
>>> unlikely(current->domain->arch.mem_access_emulate_enabled)
>>> +                  
>>> unlikely(current->domain->arch.mem_access_emulate_enabled &&
>>> +                           
>>> current->domain->arch.mem_access_emulate_each_rep)
>>>                             ? 1 : 4096);
>> 
>> unlikely() should not wrap compound conditions, or else its effect of
>> eliminating mis-predicted branches from the fast path won't be
>> achieved. In the case here I wonder though whether you couldn't
>> simply test only ->arch.mem_access_emulate_each_rep.
> 
> I'll unfold the unlikely().
> 
> Testing only ->arch.mem_access_emulate_each_rep is what I had done in
> the original patch, however on Andrew Cooper's suggestion I've now gated
> this on ->domain->arch.mem_access_emulate_enabled as well.
> 
> Otherwise, somebody might set mem_access_emulate_each_rep via its
> xc_monitor_*() call, but then after calling xc_monitor_disable() it
> would still be in effect, even if the guest is no longer being monitored.
> 
> If this is not a problem, I'm happy to check just
> ->arch.mem_access_emulate_each_rep.

Or perhaps "disabled" should just clear that flag, also to not surprise
the next one to "enable"?

Jan


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