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

Zachary Amsden
2010-03-17T21:14:52Z

On 03/16/2010 11:28 PM, Sheng Yang wrote:
> On Wednesday 17 March 2010 10:34:33 Zhang, Yanmin wrote:
>
>> On Tue, 2010-03-16 at 11:32 +0200, Avi Kivity wrote:
>>
>>> On 03/16/2010 09:48 AM, Zhang, Yanmin wrote:
>>>
>>>> Right, but there is a scope between kvm_guest_enter and really running
>>>> in guest os, where a perf event might overflow. Anyway, the scope is
>>>> very narrow, I will change it to use flag PF_VCPU.
>>>>
>>> There is also a window between setting the flag and calling 'int $2'
>>> where an NMI might happen and be accounted incorrectly.
>>>
>>> Perhaps separate the 'int $2' into a direct call into perf and another
>>> call for the rest of NMI handling. I don't see how it would work on svm
>>> though - AFAICT the NMI is held whereas vmx swallows it.
>>>
>>> I guess NMIs
>>> will be disabled until the next IRET so it isn't racy, just tricky.
>>>
>> 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.
>
> 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...
>

You can't use the APIC to send vectors 0x00-0x1f, or at least, aren't
supposed to be able to.

Zach