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

Avi Kivity
2007-10-26T09:58:51Z

Dong, Eddie wrote:
>>>
>> 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.
>>
>
> Faster? maybe.
> The issue is clflush take va parameter. So KVM needs to map gpa first
> and
> then do flush.
>
> WIth this additional overhead. I am not sure which one is faster. But
> yes,
> this is the trend we may walk toward to reduce Deny of Service.
> (flush host or other VM's cache will slowdown whole system).
>

The issue is not total time to execute (wbinvd is probably faster), but
the time where interrupts are blocked. wbinvd can block interrupts for
milliseconds, and if your industrial control machine needs service every
100 microsecond, something breaks.

[Background: this is for running Linux with realtime extensions as host,
with realtime processes on the host doing the control tasks and a guest
doing the GUI and perhaps communications.]

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