Re: High vm-exit latencies during kvm boot-up/shutdown

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Dong, Eddie wrote:
> Avi Kivity wrote:
>   
>> Dong, Eddie wrote:
>>     
>>>> There's a two-liner required to make it work.  I'll add it soon.
>>>>
>>>>
>>>>         
>>> But you still needs to issue WBINVD to all pCPUs which just move
>>> non-response time from one place to another, not?
>>>
>>>       
>> You don't actually need to emulate wbinvd, you can just ignore it.
>>
>> The only reason I can think of to use wbinvd is if you're taking a cpu
>> down (for a deep sleep state, or if you're shutting it off).  A guest
>> need not do that. 
>>
>> Any other reason? dma?  all dma today is cache-coherent, no?
>>
>>     
> For legacy PCI device, yes it is cache-cohetent, but for PCIe devices,
> it is no longer a must. A PCIe device may not generate snoopy cycle
> and thus require OS to flush cache.
>
> For example, a guest with direct device, say audio, can copy 
> data to dma buffer and then wbinvd to flush cache out and ask HW 
> DMA to output.
>   

Okay.  In that case the host can emulate wbinvd by using the clflush 
instruction, which is much faster (although overall execution time may 
be higher), maintaining real-time response times.

-- 
error compiling committee.c: too many arguments to function


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