linux-kernel - Re: Problem with kvm on -tip

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Date:	Sun, 12 Apr 2009 14:53:22 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Luis Henriques <henrix@...o.pt>
CC:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: Problem with kvm on -tip

Luis Henriques wrote:
> Hi,
>
> On Sat, Apr 11, 2009 at 03:08:55PM +0300, Avi Kivity wrote:
>   
>> This might be fixed by the attached patch.
>>     
>
> I confirm that the patch you sent removes the warnings but does it really solve
> the issue? (Sorry, I really do not know this code so I might be saying something
> really stupid.)
>   

It does.  If we are later migrated to another cpu, this code snippet 
will be called again and re-set the clock.

> What I understand from your patch is that the only portion of code that needs
> protection is the __get_cpu_var().  If this is true then a patch like the one
> below would do a better job.  But I am not sure that nothing else needs
> protection since the code immediately following the preempt_enable (in your
> patch) is an invocation to local_irq_save()...
>
> ---
>  arch/x86/kvm/x86.c |   10 +++++++---
>  1 files changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8ca100a..cf918b5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -626,13 +626,17 @@ static void kvm_write_guest_time(struct kvm_vcpu *v)
>  	unsigned long flags;
>  	struct kvm_vcpu_arch *vcpu = &v->arch;
>  	void *shared_kaddr;
> +	uint32_t tsc_khz;
>  
>  	if ((!vcpu->time_page))
>  		return;
>  
> -	if (unlikely(vcpu->hv_clock_tsc_khz != __get_cpu_var(cpu_tsc_khz))) {
> -		kvm_set_time_scale(__get_cpu_var(cpu_tsc_khz), &vcpu->hv_clock);
> -		vcpu->hv_clock_tsc_khz = __get_cpu_var(cpu_tsc_khz);
> +	preempt_disable();
> +	tsc_khz = __get_cpu_var(cpu_tsc_khz);
> +	preempt_enable();
> +	if (unlikely(vcpu->hv_clock_tsc_khz != tsc_khz)) {
> +		kvm_set_time_scale(tsc_khz, &vcpu->hv_clock);
> +		vcpu->hv_clock_tsc_khz = tsc_khz;
>  	}

Since the whole thing is unlikely(), there will be no runtime difference 
between the two patches.

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

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