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

Avi Kivity
2010-04-14T09:20:15Z

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