Re: [PATCH] KVM: x86: optimize steal time calculation

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/03/2016 12:33, Liang Chen wrote:
> Since accumulate_steal_time is now only called in record_steal_time, it
> doesn't quite make sense to put the delta calculation in a separate
> function. The function could be called thousands of times before guest
> enables the steal time MSR (though the compiler may optimize out this
> function call). And after it's enabled, the MSR enable bit is tested twice
> every time. Removing the accumulate_steal_time function also avoids the
> necessity of having the accum_steal field.
> 
> Signed-off-by: Liang Chen <liangchen.linux@gmail.com>
> Signed-off-by: Gavin Guo <gavin.guo@canonical.com>
> ---
>  arch/x86/include/asm/kvm_host.h |  1 -
>  arch/x86/kvm/x86.c              | 19 +++----------------
>  2 files changed, 3 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 44adbb8..3f460a7 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -547,7 +547,6 @@ struct kvm_vcpu_arch {
>  	struct {
>  		u64 msr_val;
>  		u64 last_steal;
> -		u64 accum_steal;
>  		struct gfn_to_hva_cache stime;
>  		struct kvm_steal_time steal;
>  	} st;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index eaf6ee8..82bfefc 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2002,22 +2002,8 @@ static void kvmclock_reset(struct kvm_vcpu *vcpu)
>  	vcpu->arch.pv_time_enabled = false;
>  }
>  
> -static void accumulate_steal_time(struct kvm_vcpu *vcpu)
> -{
> -	u64 delta;
> -
> -	if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED))
> -		return;
> -
> -	delta = current->sched_info.run_delay - vcpu->arch.st.last_steal;
> -	vcpu->arch.st.last_steal = current->sched_info.run_delay;
> -	vcpu->arch.st.accum_steal = delta;
> -}
> -
>  static void record_steal_time(struct kvm_vcpu *vcpu)
>  {
> -	accumulate_steal_time(vcpu);
> -
>  	if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED))
>  		return;
>  
> @@ -2025,9 +2011,10 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
>  		&vcpu->arch.st.steal, sizeof(struct kvm_steal_time))))
>  		return;
>  
> -	vcpu->arch.st.steal.steal += vcpu->arch.st.accum_steal;
> +	vcpu->arch.st.steal.steal += current->sched_info.run_delay -
> +		vcpu->arch.st.last_steal;
> +	vcpu->arch.st.last_steal = current->sched_info.run_delay;
>  	vcpu->arch.st.steal.version += 2;
> -	vcpu->arch.st.accum_steal = 0;
>  
>  	kvm_write_guest_cached(vcpu->kvm, &vcpu->arch.st.stime,
>  		&vcpu->arch.st.steal, sizeof(struct kvm_steal_time));
> 

Nice cleanup; queued for 4.7, thanks.

Paolo