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

[Paolo Bonzini]

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@xxxxxxxxxx>
> ---
>  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