linux-kernel - RE: A question about vring operation

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Date:	Fri, 28 Aug 2015 05:25:41 +0000
From:	"Wang, Wei W" <wei.w.wang@...el.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: RE: A question about vring operation


On 24/07/2015 21:36,  Michael S. Tsirkin wrote:
>On Thu, Aug 27, 2015 at 01:18:47AM +0000, Wang, Wei W wrote:
> Hi all,
> 
>  
> 
> I have a question about the vring_avail:
> 
> It only includes an idx (equivalent to the ring tail), which is used 
> by the frontend (virtio_net) to fill bufs. The backend (e.g. 
> vhost_net) maintains the ring head (last_avail_idx) by itself. The 
> frontend checks if the ring is full or empty via a counter (vq->num_free).
> 
> My question is why can’t we include the ring head in the  vring_avail 
> struct, so that the vq->num_free is not needed, and the backend can 
> directly use it without maintaining its own copy?
> 
>  
> 
> Thanks,
> 
> Wei
> 
>  
> 
>I'm not sure I understand your proposal, and what it would accomplish. Write a patch, that'll make it easier to discuss.
>Also copy all relevant mailing lists, not just kvm.

Thanks Michael. I haven’t got the patch ready yet. I am just wondering if it is better to have "last_avail_idx" in vhost_virtqueue moved into vring_avail.

In other regular ring operations, we usually use *head and *tail to judge if the ring is full or empty, but it seems vring_avail does not use this method in virtio_net.c (it uses a counter). 

Best,
Wei