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

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Message-ID: <4BC588CF.5010507@redhat.com>
Date:	Wed, 14 Apr 2010 12:20:15 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
CC:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Sheng Yang <sheng@...ux.intel.com>,
	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 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?

> +
> 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.

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

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