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Date: Wed, 17 Mar 2010 11:41:58 +0200
From: Avi Kivity <avi@redhat.com>
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To: Sheng Yang <sheng@linux.intel.com>
CC: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
       Ingo Molnar <mingo@elte.hu>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
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       Marcelo Tosatti <mtosatti@redhat.com>, oerg Roedel <joro@8bytes.org>,
       Jes Sorensen <Jes.Sorensen@redhat.com>, Gleb Natapov <gleb@redhat.com>,
       Zachary Amsden <zamsden@redhat.com>,
       "Huang, Zhiteng" <zhiteng.huang@intel.com>,
       Joerg Roedel <joerg.roedel@amd.com>
Subject: Re: [PATCH] Enhance perf to collect KVM guest os statistics from
 host side
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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.

-- 
error compiling committee.c: too many arguments to function