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 17:06:26 +0200 Message-ID: <20121218150626.GC27400@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> <20121218135938.GG26110@redhat.com> <50D07E6F.6040701@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: <50D07E6F.6040701@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 03:32:15PM +0100, Paolo Bonzini wrote: > Il 18/12/2012 14:59, Michael S. Tsirkin ha scritto: > >>> 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? > > virtqueue_add_buf could be slower if it used the new API. That's > because of the overhead of writing and reading from struct > virtqueue_buf, instead of using variables in registers. Yes but we'll get rid of virtqueue_buf. > >> On the other hand moving it there would eliminate the dependency > >> on virtio_ring.h. Rusty, what do you think? > >> > >>> 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. > > > > Try adding a _single variant. You might see unrolling a loop gives > > more of a benefit than this whole optimization. > > Makes sense, I'll try. However, note that I *do* need the > infrastructure in this patch because virtio-scsi could never use a > hypothetical virtqueue_add_buf_single; requests always have at least 2 > buffers for the headers. > > However I could add virtqueue_add_sg_single and use it for those > headers. Right. > The I/O buffer can keep using virtqueue_add_sg. > > Paolo