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

Dong, Eddie
2007-10-23T07:46:34Z

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>-----Original Message-----
>From: jan.kiszka-S0/GAf8tV78@public.gmane.org [mailto:jan.kiszka-S0/GAf8tV78@public.gmane.org]
>Sent: 2007年10月23日 14:38
>To: Dong, Eddie
>Cc: kvm-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org
>Subject: Re: [kvm-devel] High vm-exit latencies during kvm
>boot-up/shutdown
>
>Dong, Eddie wrote:
>> kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org wrote:
>>> Hi,
>>>
>>> I'm seeing fairly high vm-exit latencies (300-400 us)
>during and only
>>> during qemu/kvm startup and shutdown on a Core2 T5500 in
>32-bit mode.
>>> It's most probably while the VM runs inside bios code.
>During the rest
>>> of the time, while some Linux guest is running, the exit
>latencies are
>>> within microseconds, thus perfectly fine for the real-time scenarios
>>> I have in mind.
>>
>> How is this time spent? All in Qemu?
>
>Most probably. I have a function tracer installed, and it does not
>report any kernel function call between the begin of the asm block and
>its end.

If the time is spent in Qemu, then it is possible to be that long
since Qemu may read/write disk for the VM virtual disk.

>
>> Usually a kernel only VM Exit cost less than 1us.
>
>That's what I'm seeing for the rest as well.
>
>I have read that certain guest states do not allow preemptions by
>external interrupts (here it is the timer IRQ), but both
>GUEST_INTERRUPTIBILITY_INFO and GUEST_ACTIVITY_STATE are 0 on entry,
>e.g. Is there a way for the guest to trigger a non-preemptible SMM
>entry, and that without the kernel noticing it?
>
I am not aware of this possibility.
Eddie

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