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

Jan Kiszka
2007-10-23T09:08:42Z

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Dong, Eddie wrote:
>
>
>> -----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.

Clarification: I can't precisely tell what code is executed in VM mode,
as I don't have qemu or that guest instrumented. I just see the kernel
entering VM mode and leaving it again more than 300 us later. So I
wonder why this is allowed while some external IRQ is pending.

>
>>> 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

Jan

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