From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rusty Russell Subject: Re: [PATCH v2 1/5] virtio: add functions for piecewise addition of buffers Date: Mon, 07 Jan 2013 10:32:19 +1030 Message-ID: <87pq1h508k.fsf@rustcorp.com.au> References: <1355833972-20319-1-git-send-email-pbonzini@redhat.com> <1355833972-20319-2-git-send-email-pbonzini@redhat.com> <87r4m45g88.fsf@rustcorp.com.au> <50E54DC0.4040609@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <50E54DC0.4040609@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: kvm@vger.kernel.org, linux-scsi@vger.kernel.org, mst@redhat.com, hutao@cn.fujitsu.com, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, stefanha@redhat.com, Jens Axboe List-Id: linux-scsi@vger.kernel.org Paolo Bonzini writes: > Il 02/01/2013 06:03, Rusty Russell ha scritto: >> 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; >> >> Chained scatterlists are a horrible interface, but that doesn't mean we >> shouldn't support them if there's a need. >> >> I think I once even had a patch which passed two chained sgs, rather >> than a combo sg and two length numbers. It's very old, but I've pasted >> it below. >> >> Duplicating the implementation by having another interface is pretty >> nasty; I think I'd prefer the chained scatterlists, if that's optimal >> for you. > > Unfortunately, that cannot work because not all architectures support > chained scatterlists. WHAT? I can't figure out what an arch needs to do to support this? Why is it an option for archs? Why is sg_chain() even defined for non-ARCH_HAS_SG_CHAIN? Jens, help!! All archs we care about support them, though, so I think we can ignore this issue for now. > (Also, as you mention chained scatterlists are horrible. They'd happen > to work for virtio-scsi, but not for virtio-blk where the response > status is part of the footer, not the header). We lost that debate 5 years ago, so we hack around it as needed. We can add helpers to append if we need. Thanks, Rusty. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753363Ab3AGBR0 (ORCPT ); Sun, 6 Jan 2013 20:17:26 -0500 Received: from ozlabs.org ([203.10.76.45]:60094 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753307Ab3AGBRW (ORCPT ); Sun, 6 Jan 2013 20:17:22 -0500 From: Rusty Russell To: Paolo Bonzini Cc: linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org, kvm@vger.kernel.org, mst@redhat.com, hutao@cn.fujitsu.com, virtualization@lists.linux-foundation.org, stefanha@redhat.com, Jens Axboe Subject: Re: [PATCH v2 1/5] virtio: add functions for piecewise addition of buffers In-Reply-To: <50E54DC0.4040609@redhat.com> References: <1355833972-20319-1-git-send-email-pbonzini@redhat.com> <1355833972-20319-2-git-send-email-pbonzini@redhat.com> <87r4m45g88.fsf@rustcorp.com.au> <50E54DC0.4040609@redhat.com> User-Agent: Notmuch/0.14 (http://notmuchmail.org) Emacs/23.4.1 (i686-pc-linux-gnu) Date: Mon, 07 Jan 2013 10:32:19 +1030 Message-ID: <87pq1h508k.fsf@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Paolo Bonzini writes: > Il 02/01/2013 06:03, Rusty Russell ha scritto: >> 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; >> >> Chained scatterlists are a horrible interface, but that doesn't mean we >> shouldn't support them if there's a need. >> >> I think I once even had a patch which passed two chained sgs, rather >> than a combo sg and two length numbers. It's very old, but I've pasted >> it below. >> >> Duplicating the implementation by having another interface is pretty >> nasty; I think I'd prefer the chained scatterlists, if that's optimal >> for you. > > Unfortunately, that cannot work because not all architectures support > chained scatterlists. WHAT? I can't figure out what an arch needs to do to support this? Why is it an option for archs? Why is sg_chain() even defined for non-ARCH_HAS_SG_CHAIN? Jens, help!! All archs we care about support them, though, so I think we can ignore this issue for now. > (Also, as you mention chained scatterlists are horrible. They'd happen > to work for virtio-scsi, but not for virtio-blk where the response > status is part of the footer, not the header). We lost that debate 5 years ago, so we hack around it as needed. We can add helpers to append if we need. Thanks, Rusty.