From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH 09/18] virtio: use avail_event index Date: Tue, 17 May 2011 09:10:31 +0300 Message-ID: <20110517061031.GC26989__33087.0058191524$1305612671$gmane$org@redhat.com> References: <8bba6a0a8eee17e741c5155b04ff1b1c9f34bf94.1304541919.git.mst@redhat.com> <874o54h4rt.fsf@rustcorp.com.au> <20110515135541.GF24932@redhat.com> <87ei3zdsq2.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: <87ei3zdsq2.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 04:42:21PM +0930, Rusty Russell wrote: > On Sun, 15 May 2011 16:55:41 +0300, "Michael S. Tsirkin" wrote: > > On Mon, May 09, 2011 at 02:03:26PM +0930, Rusty Russell wrote: > > > On Wed, 4 May 2011 23:51:47 +0300, "Michael S. Tsirkin" wrote: > > > > Use the new avail_event feature to reduce the number > > > > of exits from the guest. > > > > > > Figures here would be nice :) > > > > You mean ASCII art in comments? > > I mean benchmarks of some kind. :) > > > > > > @@ -228,6 +237,12 @@ add_head: > > > > * new available array entries. */ > > > > virtio_wmb(); > > > > vq->vring.avail->idx++; > > > > + /* If the driver never bothers to kick in a very long while, > > > > + * avail index might wrap around. If that happens, invalidate > > > > + * kicked_avail index we stored. TODO: make sure all drivers > > > > + * kick at least once in 2^16 and remove this. */ > > > > + if (unlikely(vq->vring.avail->idx == vq->kicked_avail)) > > > > + vq->kicked_avail_valid = true; > > > > > > If they don't, they're already buggy. Simply do: > > > WARN_ON(vq->vring.avail->idx == vq->kicked_avail); > > > > Hmm, but does it say that somewhere? > > AFAICT it's a corollary of: > 1) You have a finite ring of size <= 2^16. > 2) You need to kick the other side once you've done some work. Well one can imagine a driver doing: while (virtqueue_get_buf()) { virtqueue_add_buf() } virtqueue_kick() which looks sensible (batch kicks) but might process any number of bufs between kicks. If we look at drivers closely enough, I think none of them do the equivalent of the above, but not 100% sure. > > > > @@ -482,6 +517,8 @@ void vring_transport_features(struct virtio_device *vdev) > > > > break; > > > > case VIRTIO_RING_F_USED_EVENT_IDX: > > > > break; > > > > + case VIRTIO_RING_F_AVAIL_EVENT_IDX: > > > > + break; > > > > default: > > > > /* We don't understand this bit. */ > > > > clear_bit(i, vdev->features); > > > > > > Does this belong in a prior patch? > > > > > > Thanks, > > > Rusty. > > > > Well if we don't support the feature in the ring we should not > > ack the feature, right? > > Ah, you're right. > > Thanks, > Rusty.