From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756378Ab1ESHYl (ORCPT ); Thu, 19 May 2011 03:24:41 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36838 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755809Ab1ESHYk (ORCPT ); Thu, 19 May 2011 03:24:40 -0400 Date: Thu, 19 May 2011 10:24:12 +0300 From: "Michael S. Tsirkin" To: Rusty Russell Cc: linux-kernel@vger.kernel.org, Carsten Otte , Christian Borntraeger , linux390@de.ibm.com, Martin Schwidefsky , Heiko Carstens , Shirley Ma , lguest@lists.ozlabs.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-s390@vger.kernel.org, kvm@vger.kernel.org, Krishna Kumar , Tom Lendacky , steved@us.ibm.com, habanero@linux.vnet.ibm.com Subject: Re: [PATCH 14/18] virtio: add api for delayed callbacks Message-ID: <20110519072412.GA31253@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-Disposition: inline In-Reply-To: <87boz3dsoe.fsf@rustcorp.com.au> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.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 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@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 Cc: Krishna Kumar , Carsten Otte , lguest-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org, Shirley Ma , kvm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-s390-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, habanero-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org, Heiko Carstens , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, virtualization-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, steved-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org, Christian Borntraeger , Tom Lendacky , Martin Schwidefsky , linux390-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org To: Rusty Russell Return-path: Content-Disposition: inline In-Reply-To: <87boz3dsoe.fsf-8n+1lVoiYb80n/F98K4Iww@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: lguest-bounces+glkvl-lguest=m.gmane.org-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org Sender: lguest-bounces+glkvl-lguest=m.gmane.org-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org List-Id: netdev.vger.kernel.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