linux-kernel - Re: [PATCH] ringtest: Look at next descriptor before setting up event index

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Message-ID: <4cf8d7ac-c34e-2cd3-c552-37daa41467c8@redhat.com>
Date:   Tue, 4 Oct 2016 00:48:15 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     mst@...hat.com
Subject: Re: [PATCH] ringtest: Look at next descriptor before setting up event
 index



On 19/09/2016 13:38, Paolo Bonzini wrote:
> This avoids an expensive memory barrier if the next descriptor
> is already there.  The benefit is around 10%.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  tools/virtio/ringtest/ring.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/tools/virtio/ringtest/ring.c b/tools/virtio/ringtest/ring.c
> index c25c8d248b6b..e617e20538bf 100644
> --- a/tools/virtio/ringtest/ring.c
> +++ b/tools/virtio/ringtest/ring.c
> @@ -182,6 +182,9 @@ bool enable_call()
>  {
>  	unsigned head = (ring_size - 1) & guest.last_used_idx;
>  
> +	if (!(ring[head].flags & DESC_HW))
> +		return false;
> +
>  	event->call_index = guest.last_used_idx;
>  	/* Flush call index write */
>  	/* Barrier D (for pairing) */
> @@ -215,6 +218,9 @@ bool enable_kick()
>  {
>  	unsigned head = (ring_size - 1) & host.used_idx;
>  
> +	if (ring[head].flags & DESC_HW)
> +		return false;
> +
>  	event->kick_index = host.used_idx;
>  	/* Barrier C (for pairing) */
>  	smp_mb();
> 

Ping?

Paolo