From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH v2 1/5] virtio: add functions for piecewise addition of buffers Date: Tue, 18 Dec 2012 15:59:38 +0200 Message-ID: <20121218135938.GG26110@redhat.com> References: <1355833972-20319-1-git-send-email-pbonzini@redhat.com> <1355833972-20319-2-git-send-email-pbonzini@redhat.com> <20121218133606.GC26110@redhat.com> <50D07317.8050902@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: <50D07317.8050902@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: Paolo Bonzini Cc: linux-scsi@vger.kernel.org, kvm@vger.kernel.org, hutao@cn.fujitsu.com, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, stefanha@redhat.com List-Id: virtualization@lists.linuxfoundation.org On Tue, Dec 18, 2012 at 02:43:51PM +0100, Paolo Bonzini wrote: > Il 18/12/2012 14:36, Michael S. Tsirkin ha scritto: > > Some comments without arguing about whether the performance > > benefit is worth it. > > > > On Tue, Dec 18, 2012 at 01:32:48PM +0100, Paolo Bonzini wrote: > >> diff --git a/include/linux/virtio.h b/include/linux/virtio.h > >> index cf8adb1..39d56c4 100644 > >> --- a/include/linux/virtio.h > >> +++ b/include/linux/virtio.h > >> @@ -7,6 +7,7 @@ > >> #include > >> #include > >> #include > >> +#include > >> #include > >> > >> /** > >> @@ -40,6 +41,26 @@ int virtqueue_add_buf(struct virtqueue *vq, > >> void *data, > >> gfp_t gfp); > >> > >> +struct virtqueue_buf { > >> + struct virtqueue *vq; > >> + struct vring_desc *indirect, *tail; > > > > This is wrong: virtio.h does not include virito_ring.h, > > and it shouldn't by design depend on it. > > > >> + int head; > >> +}; > >> + > > > > Can't we track state internally to the virtqueue? > > Exposing it seems to buy us nothing since you can't > > call add_buf between start and end anyway. > > I wanted to keep the state for these functions separate from the rest. > I don't think it makes much sense to move it to struct virtqueue unless > virtqueue_add_buf is converted to use the new API (doesn't make much > sense, could even be a tad slower). Why would it be slower? > On the other hand moving it there would eliminate the dependency on > virtio_ring.h. Rusty, what do you think? > > >> +int virtqueue_start_buf(struct virtqueue *_vq, > >> + struct virtqueue_buf *buf, > >> + void *data, > >> + unsigned int count, > >> + unsigned int count_sg, > >> + gfp_t gfp); > >> + > >> +void virtqueue_add_sg(struct virtqueue_buf *buf, > >> + struct scatterlist sgl[], > >> + unsigned int count, > >> + enum dma_data_direction dir); > >> + > > > > And idea: in practice virtio scsi seems to always call sg_init_one, no? > > So how about we pass in void* or something and avoid using sg and count? > > This would make it useful for -net BTW. > > It also passes the scatterlist from the LLD. It calls sg_init_one for > the request/response headers. > > Paolo Try adding a _single variant. You might see unrolling a loop gives more of a benefit than this whole optimization. > >> +void virtqueue_end_buf(struct virtqueue_buf *buf); > >> + > >> void virtqueue_kick(struct virtqueue *vq); > >> > >> bool virtqueue_kick_prepare(struct virtqueue *vq); > >> -- > >> 1.7.1 > >>