[PATCH v8 8/9] KVM: arm/arm64: Avoid work when userspace iqchips are not used

[Marc Zyngier]

On Wed, 13 Dec 2017 10:46:01 +0000,
Christoffer Dall wrote:
> 
> We currently check if the VM has a userspace irqchip on every exit from
> the VCPU, and if so, we do some work to ensure correct timer behavior.
> This is unfortunate, as we could avoid doing any work entirely, if we
> didn't have to support irqchip in userspace.
> 
> Realizing the userspace irqchip on ARM is mostly a developer or hobby
> feature, and is unlikely to be used in servers or other scenarios where
> performance is a priority, we can use a refcounted static key to only
> check the irqchip configuration when we have at least one VM that uses
> an irqchip in userspace.
> 
> Reviewed-by: Eric Auger <eric.auger at redhat.com>
> Signed-off-by: Christoffer Dall <christoffer.dall at linaro.org>

On its own, this doesn't seem to be that useful. As far as I can see,
it saves us a load from the kvm structure before giving up. I think it
is more the cumulative effect of this load that could have an impact,
but you're only dealing with it at a single location.

How about making this a first class helper and redefine
irqchip_in_kernel as such:

static inline bool irqchip_in_kernel(struct kvm *kvm)
{
	if (static_branch_unlikely(&userspace_irqchip_in_use) &&
	    unlikely(!irqchip_in_kernel(kvm)))
		return true;

	return false;
}

and move that static key to a more central location?

> ---
>  virt/kvm/arm/arch_timer.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> index f8d09665ddce..73d262c4712b 100644
> --- a/virt/kvm/arm/arch_timer.c
> +++ b/virt/kvm/arm/arch_timer.c
> @@ -51,6 +51,8 @@ static void kvm_timer_update_irq(struct kvm_vcpu *vcpu, bool new_level,
>  				 struct arch_timer_context *timer_ctx);
>  static bool kvm_timer_should_fire(struct arch_timer_context *timer_ctx);
>  
> +static DEFINE_STATIC_KEY_FALSE(userspace_irqchip_in_use);
> +
>  u64 kvm_phys_timer_read(void)
>  {
>  	return timecounter->cc->read(timecounter->cc);
> @@ -562,7 +564,8 @@ static void unmask_vtimer_irq_user(struct kvm_vcpu *vcpu)
>  
>  void kvm_timer_sync_hwstate(struct kvm_vcpu *vcpu)
>  {
> -	unmask_vtimer_irq_user(vcpu);
> +	if (static_branch_unlikely(&userspace_irqchip_in_use))
> +		unmask_vtimer_irq_user(vcpu);
>  }
>  
>  int kvm_timer_vcpu_reset(struct kvm_vcpu *vcpu)
> @@ -767,6 +770,8 @@ void kvm_timer_vcpu_terminate(struct kvm_vcpu *vcpu)
>  	soft_timer_cancel(&timer->bg_timer, &timer->expired);
>  	soft_timer_cancel(&timer->phys_timer, NULL);
>  	kvm_vgic_unmap_phys_irq(vcpu, vtimer->irq.irq);
> +	if (timer->enabled && !irqchip_in_kernel(vcpu->kvm))
> +		static_branch_dec(&userspace_irqchip_in_use);
>  }
>  
>  static bool timer_irqs_are_valid(struct kvm_vcpu *vcpu)
> @@ -819,8 +824,10 @@ int kvm_timer_enable(struct kvm_vcpu *vcpu)
>  		return 0;
>  
>  	/* Without a VGIC we do not map virtual IRQs to physical IRQs */
> -	if (!irqchip_in_kernel(vcpu->kvm))
> +	if (!irqchip_in_kernel(vcpu->kvm)) {
> +		static_branch_inc(&userspace_irqchip_in_use);
>  		goto no_vgic;
> +	}
>  
>  	if (!vgic_initialized(vcpu->kvm))
>  		return -ENODEV;
> -- 
> 2.14.2
> 

Thanks,

	M.