Re: [PATCH] Enhance perf to collect KVM guest os statistics from host side

Avi Kivity
2010-03-17T09:41:58Z

On 03/17/2010 11:28 AM, Sheng Yang wrote:
>
>> I'm not sure if vmexit does break NMI context or not. Hardware NMI context
>> isn't reentrant till a IRET. YangSheng would like to double check it.
>>
> After more check, I think VMX won't remained NMI block state for host. That's
> means, if NMI happened and processor is in VMX non-root mode, it would only
> result in VMExit, with a reason indicate that it's due to NMI happened, but no
> more state change in the host.
>
> So in that meaning, there _is_ a window between VMExit and KVM handle the NMI.
> Moreover, I think we _can't_ stop the re-entrance of NMI handling code because
> "int $2" don't have effect to block following NMI.
>

That's pretty bad, as NMI runs on a separate stack (via IST). So if
another NMI happens while our int $2 is running, the stack will be
corrupted.

> And if the NMI sequence is not important(I think so), then we need to generate
> a real NMI in current vmexit-after code. Seems let APIC send a NMI IPI to
> itself is a good idea.
>
> I am debugging a patch based on apic->send_IPI_self(NMI_VECTOR) to replace
> "int $2". Something unexpected is happening...
>

I think you need DM_NMI for that to work correctly.

An alternative is to call the NMI handler directly.

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