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: Fri, 08 Feb 2013 14:35:55 +1030 Message-ID: <8738x7ih5o.fsf@rustcorp.com.au> References: <1360239752-2470-1-git-send-email-pbonzini@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: Jens Axboe , kvm@vger.kernel.org, mst@redhat.com, virtualization@lists.linux-foundation.org To: Paolo Bonzini , linux-kernel@vger.kernel.org Return-path: In-Reply-To: <1360239752-2470-1-git-send-email-pbonzini@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 List-Id: kvm.vger.kernel.org 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; }; 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? Cheers, Rusty.