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

[Jan Kiszka <jan.kiszka-S0/GAf8tV78@public.gmane.org>]

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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.

> 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?

Jan


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