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

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

Jan Kiszka wrote:
> 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.
>
>   

Which asm block?

>> 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?
>
>   


Which timer interrupt?  The host timer interrupt or the guest timer
interrupt?

Try to be much more specific in your descriptions, including what you
mean by vm exit and exactly how you measured it.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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