linux-kernel - Re: [PATCH V3] perf & kvm: Enhance perf to collect KVM guest os statistics from host side

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Date:	Wed, 14 Apr 2010 17:43:32 +0800
From:	Sheng Yang <sheng@...ux.intel.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	oerg Roedel <joro@...tes.org>,
	Jes Sorensen <Jes.Sorensen@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>, zhiteng.huang@...el.com,
	tim.c.chen@...el.com, Arnaldo Carvalho de Melo <acme@...radead.org>
Subject: Re: [PATCH V3] perf & kvm: Enhance perf to collect KVM guest os statistics from host side

On Wednesday 14 April 2010 17:20:15 Avi Kivity wrote:
> On 04/14/2030 12:05 PM, Zhang, Yanmin wrote:
> > Here is the new patch of V3 against tip/master of April 13th
> > if anyone wants to try it.
> 
> Thanks for persisting despite the flames.
> 
> Can you please separate arch/x86/kvm part of the patch?  That will make
> for easier reviewing, and will need to go through separate trees.
> 
> Sheng, did you make any progress with the NMI injection issue?

Yes, though some other works interrupt me lately...

The very first version has issue due to SELF_IPI mode can't be used to send 
NMI according to SDM. That's the reason why x2apic don't have way to do this.

But later I found another issue of fail to inspect inside the guest. I think 
it's due to NMI is asynchronous event, though it should be triggered very 
quickly, you can't guarantee that the handler would be triggered before the 
state(current_vcpu) is cleared with current code.

Maybe just extended the "guest state" region would be fine, if the latency is 
stable enough(though I think it maybe platform depended). I am working on this 
now. 

-- 
regards
Yang, Sheng

> 
> > +
> > diff -Nraup linux-2.6_tip0413/arch/x86/kvm/x86.c
> > linux-2.6_tip0413_perfkvm/arch/x86/kvm/x86.c ---
> > linux-2.6_tip0413/arch/x86/kvm/x86.c	2010-04-14 11:11:04.341042024 +0800
> > +++ linux-2.6_tip0413_perfkvm/arch/x86/kvm/x86.c	2010-04-14
> > 11:32:45.841278890 +0800 @@ -3765,6 +3765,35 @@ static void
> > kvm_timer_init(void)
> >   	}
> >   }
> >
> > +static DEFINE_PER_CPU(struct kvm_vcpu *, current_vcpu);
> > +
> > +static int kvm_is_in_guest(void)
> > +{
> > +	return percpu_read(current_vcpu) != NULL;
> 
> An even more accurate way to determine this is to check whether the
> interrupt frame points back at the 'int $2' instruction.  However we
> plan to switch to a self-IPI method to inject the NMI, and I'm not sure
> wether APIC NMIs are accepted on an instruction boundary or whether
> there's some latency involved.
> 
> > +static unsigned long kvm_get_guest_ip(void)
> > +{
> > +	unsigned long ip = 0;
> > +	if (percpu_read(current_vcpu))
> > +		ip = kvm_rip_read(percpu_read(current_vcpu));
> > +	return ip;
> > +}
> 
> This may be racy.  kvm_rip_read() accesses a cache in memory; if we're
> in the process of updating the cache, then we may read a stale value.
> See below.
> 
> >   	trace_kvm_entry(vcpu->vcpu_id);
> > +
> > +	percpu_write(current_vcpu, vcpu);
> >   	kvm_x86_ops->run(vcpu);
> > +	percpu_write(current_vcpu, NULL);
> 
> If you move this around the 'int $2' instructions you will close the
> race, as a stray NMI won't catch us updating the rip cache.  But that
> depends on whether self-IPI is accepted on the next instruction or not.
> 
--
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