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

[Zachary Amsden]

On 03/17/2010 07:41 PM, Sheng Yang wrote:
> 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.
>    

Yes, I think we agree.  NMI is the only vector in 0x0-0xf which can be 
sent via self-IPI because the vector itself does not matter for NMI.

Zach
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