From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [RFC PATCH] virtio_console: link vq to port with a private
	pointer in struct virtqueue
Date: Tue, 8 May 2012 13:18:39 +0300
Message-ID: <20120508101839.GA15598@redhat.com>
References: <87397be80k.fsf@rustcorp.com.au>
	<1336470355-19257-1-git-send-email-pbonzini@redhat.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Return-path: <virtualization-bounces@lists.linux-foundation.org>
Content-Disposition: inline
In-Reply-To: <1336470355-19257-1-git-send-email-pbonzini@redhat.com>
List-Unsubscribe: <https://lists.linuxfoundation.org/mailman/options/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=unsubscribe>
List-Archive: <http://lists.linuxfoundation.org/pipermail/virtualization/>
List-Post: <mailto:virtualization@lists.linux-foundation.org>
List-Help: <mailto:virtualization-request@lists.linux-foundation.org?subject=help>
List-Subscribe: <https://lists.linuxfoundation.org/mailman/listinfo/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=subscribe>
Sender: virtualization-bounces@lists.linux-foundation.org
Errors-To: virtualization-bounces@lists.linux-foundation.org
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Amit Shah <amit.shah@redhat.com>, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org
List-Id: virtualization@lists.linuxfoundation.org

On Tue, May 08, 2012 at 11:45:55AM +0200, Paolo Bonzini wrote:
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -76,8 +76,6 @@
>  
>  struct vring_virtqueue
>  {
> -	struct virtqueue vq;
> -
>  	/* Actual memory layout for this queue */
>  	struct vring vring;
>  
> @@ -106,6 +104,9 @@ struct vring_virtqueue
>  	/* How to notify other side. FIXME: commonalize hcalls! */
>  	void (*notify)(struct virtqueue *vq);
>  
> +	/* Tokens for callbacks. */
> +	void **data;
> +
>  #ifdef DEBUG
>  	/* They're supposed to lock for us. */
>  	unsigned int in_use;
> @@ -115,8 +116,9 @@ struct vring_virtqueue
>  	ktime_t last_add_time;
>  #endif
>  
> -	/* Tokens for callbacks. */
> -	void *data[];
> +	struct virtqueue vq;
> +
> +	/* Bus-specific virtqueue data goes here.  */
>  };
>  
>  #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)


This moves the data out of line and the bus specific stuff inline.
But bus accesses only happen on io and interrupt which are already
slow, while data accesses happen on fast path.

>From that POV this looks like a wrong thing to do.

-- 
MST