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

Dong, Eddie
2007-10-26T08:32:19Z

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.

Thx,Eddie