Re: [PATCH v5] KVM: VMX: Execute WBINVD to keep data consistency with assigned devices

[Avi Kivity <avi@redhat.com>]

On 06/29/2010 01:32 PM, Jan Kiszka wrote:
> Avi Kivity wrote:
>    
>> On 06/29/2010 06:16 AM, Sheng Yang wrote:
>>      
>>> Some guest device driver may leverage the "Non-Snoop" I/O, and explicitly
>>> WBINVD or CLFLUSH to a RAM space. Since migration may occur before WBINVD or
>>> CLFLUSH, we need to maintain data consistency either by:
>>> 1: flushing cache (wbinvd) when the guest is scheduled out if there is no
>>> wbinvd exit, or
>>> 2: execute wbinvd on all dirty physical CPUs when guest wbinvd exits.
>>>
>>>
>>>        
>> Looks good.
>>
>>      
> There is just the question remaining if we want to add some disable
> knob, maybe as an option in the device assignment configuration.
>    

Patches welcome.  Also an alternative implementation that uses clflush 
could also work.

> I wonder what the performance impact of this feature is when using CPUs
> without wbinvd exiting. Can we afford to enable it unconditionally (in
> the presence of pass-through) even if the guest doesn't need it?
>    

Correctness is more important than performance.  Since we don't know 
whether the guest needs it or not, we have to enable it.  The user may 
disable it if he likes.

Maybe we should emit a warning that performance may degrade when pci 
snooping is not available.

-- 
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