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

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Date:	Thu, 18 Mar 2010 13:41:15 +0800
From:	Sheng Yang <sheng@...ux.intel.com>
To:	kvm@...r.kernel.org
Cc:	Zachary Amsden <zamsden@...hat.com>, Avi Kivity <avi@...hat.com>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	oerg Roedel <joro@...tes.org>,
	Jes Sorensen <Jes.Sorensen@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	"Huang, Zhiteng" <zhiteng.huang@...el.com>,
	Joerg Roedel <joerg.roedel@....com>
Subject: Re: [PATCH] Enhance perf to collect KVM guest os statistics from host side

On Thursday 18 March 2010 13:22:28 Sheng Yang wrote:
> On Thursday 18 March 2010 12:50:58 Zachary Amsden wrote:
> > On 03/17/2010 03:19 PM, Sheng Yang wrote:
> > > On Thursday 18 March 2010 05:14:52 Zachary Amsden wrote:
> > >> 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.
> > >
> > > Um? Why?
> > >
> > > Especially kernel is already using it to deliver NMI.
> >
> > That's the only defined case, and it is defined because the vector field
> > is ignore for DM_NMI.  Vol 3A (exact section numbers may vary depending
> > on your version).
> >
> > 8.5.1 / 8.6.1
> >
> > '100 (NMI) Delivers an NMI interrupt to the target processor or
> > processors.  The vector information is ignored'
> >
> > 8.5.2  Valid Interrupt Vectors
> >
> > 'Local and I/O APICs support 240 of these vectors (in the range of 16 to
> > 255) as valid interrupts.'
> >
> > 8.8.4 Interrupt Acceptance for Fixed Interrupts
> >
> > '...; vectors 0 through 15 are reserved by the APIC (see also: Section
> > 8.5.2, "Valid Interrupt Vectors")'
> >
> > So I misremembered, apparently you can deliver interrupts 0x10-0x1f, but
> > vectors 0x00-0x0f are not valid to send via APIC or I/O APIC.
> 
> As you pointed out, NMI is not "Fixed interrupt". If we want to send NMI,
>  it would need a specific delivery mode rather than vector number.
> 
> And if you look at code, if we specific NMI_VECTOR, the delivery mode would
>  be set to NMI.
> 
> So what's wrong here?

OK, I think I understand your points now. You meant that these vectors can't 
be filled in vector field directly, right? But NMI is a exception due to 
DM_NMI. Is that your point? I think we agree on this.

-- 
regards
Yang, Sheng
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