Re: [PATCH v3] KVM: VMX: Execute WBINVD to keep data consistency with assigned devices

[Avi Kivity <avi@redhat.com>]

On 06/28/2010 09:56 AM, Sheng Yang wrote:
> On Monday 28 June 2010 14:56:38 Avi Kivity wrote:
>    
>> On 06/28/2010 09:42 AM, Sheng Yang wrote:
>>      
>>>>> +static void wbinvd_ipi(void *garbage)
>>>>> +{
>>>>> +	wbinvd();
>>>>> +}
>>>>>            
>>>> Like Jan mentioned, this is quite heavy.  What about a clflush() loop
>>>> instead?  That may take more time, but at least it's preemptible.  Of
>>>> course, it isn't preemptible in an IPI.
>>>>          
>>> I think this kind of behavior happened rarely, and most recent processor
>>> should have WBINVD exit which means it's an IPI... So I think it's maybe
>>> acceptable here.
>>>        
>> Several milliseconds of non-responsiveness may not be acceptable for
>> some applications.  So I think queue_work_on() and a clflush loop is
>> better than an IPI and wbinvd.
>>      
> OK... Would update it in the next version.
>
>    

Hm, the manual says (regarding clflush):

> Invalidates the cache line that contains the linear address specified 
> with the source
> operand from all levels of the processor cache hierarchy (data and 
> instruction). The
> invalidation is broadcast throughout the cache coherence domain. If, 
> at any level of
> the cache hierarchy, the line is inconsistent with memory (dirty) it 
> is written to
> memory before invalidation.

So I don't think you need to queue_work_on(), instead you can work in 
vcpu thread context.  But better check with someone that really knows.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.