[PATCH 9/9] KVM: arm/arm64: vgic: Improve sync_hwstate performance

[Marc Zyngier]

On 20/03/17 10:58, Christoffer Dall wrote:
> There is no need to call any functions to fold LRs when we don't use any
> LRs and we don't need to mess with overflow flags, take spinlocks, or
> prune the AP list if the AP list is empty.
> 
> Note: list_empty is a single atomic read (uses READ_ONCE) and can
> therefore check if a list is empty or not without the need to take the
> spinlock protecting the list.
> 
> Signed-off-by: Christoffer Dall <cdall at linaro.org>
> ---
>  virt/kvm/arm/vgic/vgic.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/virt/kvm/arm/vgic/vgic.c b/virt/kvm/arm/vgic/vgic.c
> index 093873e..8ecb009 100644
> --- a/virt/kvm/arm/vgic/vgic.c
> +++ b/virt/kvm/arm/vgic/vgic.c
> @@ -639,15 +639,18 @@ void kvm_vgic_sync_hwstate(struct kvm_vcpu *vcpu)
>  {
>  	struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
>  
> -	if (unlikely(!vgic_initialized(vcpu->kvm)))

Could this be folded with the previous patch? It is the same optimisation.

> +	/* An empty ap_list_head implies used_lrs == 0 */
> +	if (list_empty(&vcpu->arch.vgic_cpu.ap_list_head))
>  		return;
>  
>  	vgic_clear_uie(vcpu);
> -	vgic_fold_lr_state(vcpu);
> -	vgic_prune_ap_list(vcpu);
>  
> -	/* Make sure we can fast-path in flush_hwstate */
> -	vgic_cpu->used_lrs = 0;
> +	if (vgic_cpu->used_lrs) {
> +		vgic_fold_lr_state(vcpu);
> +		vgic_cpu->used_lrs = 0;

This zeroing could also be moved to vgic_fold_lr_state(), though I don't
feel strongly about it.

> +	}
> +
> +	vgic_prune_ap_list(vcpu);
>  }
>  
>  /* Flush our emulation state into the GIC hardware before entering the guest. */
> 

Otherwise looks good to me.

	M.
-- 
Jazz is not dead. It just smells funny...