From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rusty Russell Subject: Re: [RFC PATCH 0/8] virtio: new API for addition of buffers, scatterlist changes Date: Wed, 13 Feb 2013 20:16:42 +1030 Message-ID: <87halglf5p.fsf@rustcorp.com.au> References: <1360239752-2470-1-git-send-email-pbonzini@redhat.com> <8738x7ih5o.fsf@rustcorp.com.au> <20130208115240.GN15092@kernel.dk> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, mst@redhat.com, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Paolo Bonzini To: Jens Axboe Return-path: In-Reply-To: <20130208115240.GN15092@kernel.dk> 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 List-Id: kvm.vger.kernel.org Jens Axboe writes: > On Fri, Feb 08 2013, Rusty Russell wrote: >> Paolo Bonzini writes: >> > The virtqueue_add_buf function has two limitations: >> > >> > 1) it requires the caller to provide all the buffers in a single call; >> > >> > 2) it does not support chained scatterlists: the buffers must be >> > provided as an array of struct scatterlist. >> > >> > Because of these limitations, virtio-scsi has to copy each request into >> > a scatterlist internal to the driver. It cannot just use the one that >> > was prepared by the upper SCSI layers. >> >> Hi Paulo, >> >> Note that you've defined your problem in terms of your solution >> here. For clarity: >> >> The problem: we want to prepend and append to a scatterlist. We can't >> append, because the chained scatterlist implementation requires >> an element to be appended to join two scatterlists together. >> >> The solution: fix scatterlists by introducing struct sg_ring: >> struct sg_ring { >> struct list_head ring; >> unsigned int nents; >> unsigned int orig_nents; /* do we want to replace sg_table? */ >> struct scatterlist *sg; >> }; > > This would definitely be more flexible than the current chaining. > However: > >> The workaround: make virtio accept multiple scatterlists for a single >> buffer. >> >> There's nothing wrong with your workaround, but if other subsystems have >> the same problem we do, perhaps we should consider a broader solution? > > Do other use cases actually exist? I don't think I've come across this > requirement before, since it was introduced (6 years ago, from a cursory > look at the git logs!). Thanks Jens. OK, let's not over-solve the problem then, we'll make a virtio-specific solution. Paulo, I'll take your patches once you repost. Thanks, Rusty.