From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1946357Ab3BHLxw (ORCPT ); Fri, 8 Feb 2013 06:53:52 -0500 Received: from 173-166-109-252-newengland.hfc.comcastbusiness.net ([173.166.109.252]:41027 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1946329Ab3BHLxv (ORCPT ); Fri, 8 Feb 2013 06:53:51 -0500 Date: Fri, 8 Feb 2013 12:52:40 +0100 From: Jens Axboe To: Rusty Russell Cc: Paolo Bonzini , linux-kernel@vger.kernel.org, Wanlong Gao , asias@redhat.com, mst@redhat.com, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org Subject: Re: [RFC PATCH 0/8] virtio: new API for addition of buffers, scatterlist changes Message-ID: <20130208115240.GN15092@kernel.dk> References: <1360239752-2470-1-git-send-email-pbonzini@redhat.com> <8738x7ih5o.fsf@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8738x7ih5o.fsf@rustcorp.com.au> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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!). -- Jens Axboe