From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH 14/18] virtio: add api for delayed callbacks Date: Thu, 19 May 2011 10:24:12 +0300 Message-ID: <20110519072412.GA31253__31510.8672519729$1305789924$gmane$org@redhat.com> References: <64d47c628b3fdc0ac156aed4be182933d8bcc0db.1304541919.git.mst@redhat.com> <871v08h0vm.fsf@rustcorp.com.au> <20110515124818.GD24932@redhat.com> <87boz3dsoe.fsf@rustcorp.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <87boz3dsoe.fsf@rustcorp.com.au> 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: Rusty Russell Cc: Krishna Kumar , Carsten Otte , lguest@lists.ozlabs.org, Shirley Ma , kvm@vger.kernel.org, linux-s390@vger.kernel.org, netdev@vger.kernel.org, habanero@linux.vnet.ibm.com, Heiko Carstens , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, steved@us.ibm.com, Christian Borntraeger , Tom Lendacky , Martin Schwidefsky , linux390@de.ibm.com List-Id: virtualization@lists.linuxfoundation.org On Mon, May 16, 2011 at 04:43:21PM +0930, Rusty Russell wrote: > On Sun, 15 May 2011 15:48:18 +0300, "Michael S. Tsirkin" wrote: > > On Mon, May 09, 2011 at 03:27:33PM +0930, Rusty Russell wrote: > > > On Wed, 4 May 2011 23:52:33 +0300, "Michael S. Tsirkin" wrote: > > > > Add an API that tells the other side that callbacks > > > > should be delayed until a lot of work has been done. > > > > Implement using the new used_event feature. > > > > > > Since you're going to add a capacity query anyway, why not add the > > > threshold argument here? > > > > I thought that if we keep the API kind of generic > > there might be more of a chance that future transports > > will be able to implement it. For example, with an > > old host we can't commit to a specific index. > > No, it's always a hint anyway: you can be notified before the threshold > is reached. But best make it explicit I think. > > Cheers, > Rusty. I tried doing that and remembered the real reason I went for this API: capacity is limited by descriptor table space, not used ring space: each entry in the used ring frees up multiple entries in the descriptor ring. Thus the ring can't provide callback after capacity is N: capacity is only available after we get bufs. We could try and make the API pass in the number of freed bufs, however: - this is not really what virtio-net cares about (it cares about capacity) - if the driver passes a number > number of outstanding bufs, it will never get a callback. So to stay correct the driver will need to track number of outstanding requests. The simpler API avoids that. APIs are easy to change so I'm guessing it's not a major blocker: we can change later when e.g. block tries to pass in some kind of extra hint: we'll be smarter about how this API can change then. Right? -- MST