Re: [PATCH v3 07/10] KVM: arm/arm64: optimize VCPU RUN

[Christoffer Dall <cdall@linaro.org>]

On Wed, May 03, 2017 at 06:06:32PM +0200, Andrew Jones wrote:

nit: can you make the subject of this patch a bit more specific?

For example:  Optimize checking power_off flag in KVM_RUN

> We can make a small optimization by not checking the state of
> the power_off field on each run. This is done by treating
> power_off like pause, only checking it when we get the EXIT
> VCPU request. When a VCPU powers off another VCPU the EXIT
> request is already made, so we just need to make sure the
> request is also made on self power off. kvm_vcpu_kick() isn't
> necessary for these cases, as the VCPU would just be kicking
> itself, but we add it anyway as a self kick doesn't cost much,
> and it makes the code more future-proof.
> 
> Signed-off-by: Andrew Jones <drjones@redhat.com>
> ---
>  arch/arm/kvm/arm.c  | 16 ++++++++++------
>  arch/arm/kvm/psci.c |  2 ++
>  2 files changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index 26d9d4d72853..24bbc7671d89 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -371,6 +371,13 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>  	kvm_timer_vcpu_put(vcpu);
>  }
>  
> +static void vcpu_power_off(struct kvm_vcpu *vcpu)
> +{
> +	vcpu->arch.power_off = true;
> +	kvm_make_request(KVM_REQ_VCPU_EXIT, vcpu);
> +	kvm_vcpu_kick(vcpu);
> +}
> +
>  int kvm_arch_vcpu_ioctl_get_mpstate(struct kvm_vcpu *vcpu,
>  				    struct kvm_mp_state *mp_state)
>  {
> @@ -390,7 +397,7 @@ int kvm_arch_vcpu_ioctl_set_mpstate(struct kvm_vcpu *vcpu,
>  		vcpu->arch.power_off = false;
>  		break;
>  	case KVM_MP_STATE_STOPPED:
> -		vcpu->arch.power_off = true;
> +		vcpu_power_off(vcpu);
>  		break;
>  	default:
>  		return -EINVAL;
> @@ -626,14 +633,11 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
>  
>  		if (kvm_request_pending(vcpu)) {
>  			if (kvm_check_request(KVM_REQ_VCPU_EXIT, vcpu)) {
> -				if (vcpu->arch.pause)
> +				if (vcpu->arch.power_off || vcpu->arch.pause)
>  					vcpu_sleep(vcpu);
>  			}
>  		}
>  
> -		if (vcpu->arch.power_off)
> -			vcpu_sleep(vcpu);
> -

Hmmm, even though I just gave a reviewed-by on the pause side, I'm not
realizing that I don't think this works.  Because you're now only
checking requests in the vcpu loop, but the vcpu_sleep() function is
implemented using swait_event_interruptible(), which can wake up if you
have a pending signal for example, and then the loop can wrap around and
you can run the VCPU even though you should be paused.  Am I missing
something?

Thanks,
-Christoffer

>  		/*
>  		 * Preparing the interrupts to be injected also
>  		 * involves poking the GIC, which must be done in a
> @@ -903,7 +907,7 @@ static int kvm_arch_vcpu_ioctl_vcpu_init(struct kvm_vcpu *vcpu,
>  	 * Handle the "start in power-off" case.
>  	 */
>  	if (test_bit(KVM_ARM_VCPU_POWER_OFF, vcpu->arch.features))
> -		vcpu->arch.power_off = true;
> +		vcpu_power_off(vcpu);
>  	else
>  		vcpu->arch.power_off = false;
>  
> diff --git a/arch/arm/kvm/psci.c b/arch/arm/kvm/psci.c
> index f189d0ad30d5..4a436685c552 100644
> --- a/arch/arm/kvm/psci.c
> +++ b/arch/arm/kvm/psci.c
> @@ -65,6 +65,8 @@ static unsigned long kvm_psci_vcpu_suspend(struct kvm_vcpu *vcpu)
>  static void kvm_psci_vcpu_off(struct kvm_vcpu *vcpu)
>  {
>  	vcpu->arch.power_off = true;
> +	kvm_make_request(KVM_REQ_VCPU_EXIT, vcpu);
> +	kvm_vcpu_kick(vcpu);
>  }
>  
>  static unsigned long kvm_psci_vcpu_on(struct kvm_vcpu *source_vcpu)
> -- 
> 2.9.3
>