From: Rusty Russell Subject: Re: [PATCH RFC 2/3] virtio_net: fix tx capacity checks using new API Date: Thu, 02 Jun 2011 11:40:26 +0930 Message-ID: <87vcwphtkd.fsf@rustcorp.com.au> References: <110045d5bf250e6ed9c739f8cc6adbbf54e3cc9c.1306921434.git.mst@redhat.com> 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: "Michael S. Tsirkin" , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org Return-path: In-Reply-To: <110045d5bf250e6ed9c739f8cc6adbbf54e3cc9c.1306921434.git.mst-H+wXaHxf7aLQT0dZR+AlfA@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: kvm.vger.kernel.org On Wed, 1 Jun 2011 12:49:54 +0300, "Michael S. Tsirkin" wrote: > In the (rare) case where new descriptors are used > while virtio_net enables vq callback for the TX vq, > virtio_net uses the number of sg entries in the skb it frees to > calculate how many descriptors in the ring have just been made > available. But this value is an overestimate: with indirect buffers > each skb only uses one descriptor entry, meaning we may wake the queue > only to find we still can't transmit anything. This is a bit misleading. The value is an overestimate, but so is the requirement for 2+MAX_SKB_FRAGS, *unless* we suddenly drop into direct mode due to OOM. Thanks, Rusty.