From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: packed ring layout proposal v3 Date: Mon, 30 Oct 2017 18:30:57 +0200 Message-ID: <20171030182950-mutt-send-email-mst@kernel.org> References: <20160915223915.qjlnlvf2w7u37bu3@redhat.com> <20171025191346-mutt-send-email-mst@kernel.org> <20171029162117-mutt-send-email-mst@kernel.org> <20171030040341-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Ilya Lesokhin Cc: "virtio-dev@lists.oasis-open.org" , "virtualization@lists.linux-foundation.org" List-Id: virtualization@lists.linuxfoundation.org On Mon, Oct 30, 2017 at 06:30:56AM +0000, Ilya Lesokhin wrote: > > -----Original Message----- > > From: Michael S. Tsirkin [mailto:mst@redhat.com] > > Sent: Monday, October 30, 2017 4:09 AM > > To: Ilya Lesokhin > > Cc: virtio-dev@lists.oasis-open.org; virtualization@lists.linux-foundation.org > > Subject: Re: packed ring layout proposal v3 > > > > On Sun, Oct 29, 2017 at 02:34:56PM +0000, Ilya Lesokhin wrote: > > > > -----Original Message----- > > > > From: Michael S. Tsirkin [mailto:mst@redhat.com] > > > > Sent: Sunday, October 29, 2017 4:22 PM > > > > To: Ilya Lesokhin > > > > Cc: virtio-dev@lists.oasis-open.org; virtualization@lists.linux-foundation.org > > > > Subject: Re: packed ring layout proposal v3 > > > > > > > > If you do this whats the point of the id? Just use descriptor offset like virtio 0 > > did. > > > > > > > > > > I agree that ID is pointless when requests are completed in order. > > > > > > But I'm not sure what you mean by descriptor offset? > > > > Where the descriptor is within the ring. > > > > Using descriptor offset like virtio 0, won't work. > In virtio 0, there was no reordering in the descriptor ring, so the offset was always unique. > In the new spec, if descriptor in offset 2 completes before the descriptor in offset 1, > It can be put in offset 1, but reusing offset 1 is not yet safe. > > Also, please ignore my earlier comment about in-order completion, > It invalidates the entire discussion. > > Yes, using offsets only works if all descriptors are used and written back in order. If they are, descriptor ID isn't necessary. If they aren't, it's necessary. -- MST