On 20/03/17 10:58, Christoffer Dall wrote:
> From: Shih-Wei Li <[email protected]>
>
> We do not need to flush vgic states in each world switch unless
> there is pending IRQ queued to the vgic's ap list. We can thus reduce
> the overhead by not grabbing the spinlock and not making the extra
> function call to vgic_flush_lr_state.
>
> Signed-off-by: Shih-Wei Li <[email protected]>
> Signed-off-by: Christoffer Dall <[email protected]>
> ---
> virt/kvm/arm/vgic/vgic.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/virt/kvm/arm/vgic/vgic.c b/virt/kvm/arm/vgic/vgic.c
> index 2ac0def..1436c2e 100644
> --- a/virt/kvm/arm/vgic/vgic.c
> +++ b/virt/kvm/arm/vgic/vgic.c
> @@ -637,12 +637,17 @@ static void vgic_flush_lr_state(struct kvm_vcpu *vcpu)
> /* Sync back the hardware VGIC state into our emulation after a guest's run.
> */
> void kvm_vgic_sync_hwstate(struct kvm_vcpu *vcpu)
> {
> + struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
> +
> if (unlikely(!vgic_initialized(vcpu->kvm)))
> return;
>
> vgic_process_maintenance_interrupt(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;
> }
>
> /* Flush our emulation state into the GIC hardware before entering the
> guest. */
> @@ -651,6 +656,9 @@ void kvm_vgic_flush_hwstate(struct kvm_vcpu *vcpu)
> if (unlikely(!vgic_initialized(vcpu->kvm)))
> return;
>
> + if (list_empty(&vcpu->arch.vgic_cpu.ap_list_head))
> + return;
> +
While I can see the READ_ONCE in list_empty(), it is not completely
obvious that the race with another CPU injecting an interrupt is safe
(if I get it correctly, it will have the same effect as if it was added
right after the critical section below).
Can we have a nice comment explaining this?
> spin_lock(&vcpu->arch.vgic_cpu.ap_list_lock);
> vgic_flush_lr_state(vcpu);
> spin_unlock(&vcpu->arch.vgic_cpu.ap_list_lock);
>
Thanks,
M.
--
Jazz is not dead. It just smells funny...
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