From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH RFC 2/2] vhost: support urgent descriptors Date: Mon, 22 Sep 2014 09:55:39 +0300 Message-ID: <20140922065539.GA12057@redhat.com> References: <1404203661-7521-1-git-send-email-mst@redhat.com> <1404203661-7521-2-git-send-email-mst@redhat.com> <541BD6E9.2010707@redhat.com> <541D503B.3070507@redhat.com> <541F97CF.7000305@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <541F97CF.7000305@redhat.com> 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: Jason Wang Cc: Paolo Bonzini , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org On Mon, Sep 22, 2014 at 11:30:23AM +0800, Jason Wang wrote: > On 09/20/2014 06:00 PM, Paolo Bonzini wrote: > > Il 19/09/2014 09:10, Jason Wang ha scritto: > >>>> > >>>> - if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) { > >>>> + if (vq->urgent || !vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) { > >> So the urgent descriptor only work when event index was not enabled? > >> This seems suboptimal, we may still want to benefit from event index > >> even if urgent descriptor is used. Looks like we need return true here > >> when vq->urgent is true? > > Its ||, not &&. > > > > Without event index, all descriptors are treated as urgent. > > > > Paolo > > > > The problem is if vq->urgent is true, the patch checks > VRING_AVAIL_F_NO_INTERRUPT bit. This bit were set unconditionally in > virtqueue_enable_cb() regardless of event index feature and cleared > unconditionally in virtqueue_disable_cb(). The reverse actually, right? > So virtqueue_enable_cb() was > used to not only publish a new event index but also enable the urgent > descriptor. And virtqueue_disable_cb() disabled all interrupts including > the urgent descriptor. Guest won't get urgent interrupts by just adding > virtqueue_add_outbuf_urgent() since what it needs is to enable and > disable interrupt for !urgent descriptor. Right, we want a new API that advances event index but does not set VRING_AVAIL_F_NO_INTERRUPT. IMO still want to set VRING_AVAIL_F_NO_INTERRUPT when handling tx interrupts, to avoid interrupt storms. > Btw, not sure "urgent" is a suitable name, since interrupt is often slow > in kvm guest. And in fact virtio-net will probably use "urgent" > descriptor for those packets (e.g stream packets who can be delayed a > little bit to batch more bytes from userspace) who was not urgent > compared to other packets. > Yes but we are asking for an interrupt before event index is reached because something is waiting for the packet to be transmitted. I couldn't come up with a better name. -- MST