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

Jan Kiszka
2007-10-23T16:31:19Z

Avi Kivity wrote:
> Jan Kiszka wrote:
>>> Exiting on a pending interrupt is controlled by the vmcs word
>>> PIN_BASED_EXEC_CONTROL, bit PIN_BASED_EXT_INTR_MASK. Can you check (via
>>> vmcs_read32()) that the bit is indeed set?
>>>
>>> [if not, a guest can just enter a busy loop and kill a processor]
>>>
>>>
>> I traced it right before and after the asm block, and in all cases
>> (including those with low latency exits) it's just 0x1f, which should be
>> fine.
>>
>>
>
> Yes.
>
>> Earlier, I also checked GUEST_INTERRUPTIBILITY_INFO and
>> GUEST_ACTIVITY_STATE, but found neither some suspicious state nor a
>> difference compared to fast exits.
>>
>
> GUEST_* things are unrelated; these talk about whether the guest is
> ready to receive an interrupt, but we don't really care, do we?

Nope, of course (looks like my VMX manual interpreter still needs
improvements...).

>
> I don't have any idea why this is happening. This is completely
> unrelated to -rt issues -- this is between kvm and the hardware.
>
> Is this on a uniprocessor system? If not, can you try nosmp?

It's both: -rt tests were performed with nosmp (-rt locks up under SMP
here), and the Xenomai tests, including the last instrumentation, ran in
SMP mode. So I tend to exclude SMP effects.

Do you have some suggestion how to analyse what the guest is executing
while those IRQs are delayed? As I said, I suspect some interaction of
the Bochs BIOS is triggering the effect, because it only shows up (with
a Linux guest) before the BIOS prints the first messages.

Jan

--
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux