From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933298Ab3BLQJF (ORCPT ); Tue, 12 Feb 2013 11:09:05 -0500 Received: from mx1.redhat.com ([209.132.183.28]:32460 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933129Ab3BLQJD (ORCPT ); Tue, 12 Feb 2013 11:09:03 -0500 Date: Tue, 12 Feb 2013 18:13:26 +0200 From: "Michael S. Tsirkin" To: Paolo Bonzini Cc: linux-kernel@vger.kernel.org, Wanlong Gao , asias@redhat.com, Rusty Russell , kvm@vger.kernel.org, virtualization@lists.linux-foundation.org Subject: Re: [PATCH 1/9] virtio: add functions for piecewise addition of buffers Message-ID: <20130212161326.GB4373@redhat.com> References: <1360671815-2135-1-git-send-email-pbonzini@redhat.com> <1360671815-2135-2-git-send-email-pbonzini@redhat.com> <20130212145620.GA3392@redhat.com> <511A608B.5080007@redhat.com> <20130212154338.GA4083@redhat.com> <511A6457.80609@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <511A6457.80609@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 12, 2013 at 04:48:39PM +0100, Paolo Bonzini wrote: > Il 12/02/2013 16:43, Michael S. Tsirkin ha scritto: > > On Tue, Feb 12, 2013 at 04:32:27PM +0100, Paolo Bonzini wrote: > >> Il 12/02/2013 15:56, Michael S. Tsirkin ha scritto: > >>>>> +/** > >>>>> + * virtqueue_start_buf - start building buffer for the other end > >>>>> + * @vq: the struct virtqueue we're talking about. > >>>>> + * @data: the token identifying the buffer. > >>>>> + * @nents: the number of buffers that will be added > >>> This function starts building one buffer, number of buffers > >>> is a bit weird here. > >> > >> Ok. > >> > >>>>> + * @nsg: the number of sg lists that will be added > >>> This means number of calls to add_sg ? Not sure why this matters. > >>> How about we pass in in_num/out_num - that is total # of sg, > >>> same as add_buf? > >> > >> It is used to choose between direct and indirect. > > > > total number of in and out should be enough for this, no? > > Originally, I used nsg/nents because I wanted to use mixed direct and > indirect buffers. nsg/nents let me choose between full direct (nsg == > nents), mixed (num_free >= nsg), full indirect (num_free < nsg). Then I > had to give up because QEMU does not support it, but I still would like > to keep that open in the API. Problem is it does not seem to make sense in the API. > In this series, however, I am still using nsg to choose between direct > and indirect. I would like to use dirtect for small scatterlists, even > if they are surrounded by a request/response headers/footers. Shouldn't we base this on total number of s/g entries? I don't see why does it matter how many calls you use to build up the list. > >>>>> +/** > >>>>> + * virtqueue_add_sg - add sglist to buffer being built > >>>>> + * @_vq: the virtqueue for which the buffer is being built > >>>>> + * @sgl: the description of the buffer(s). > >>>>> + * @nents: the number of items to process in sgl > >>>>> + * @dir: whether the sgl is read or written (DMA_TO_DEVICE/DMA_FROM_DEVICE only) > >>>>> + * > >>>>> + * Note that, unlike virtqueue_add_buf, this function follows chained > >>>>> + * scatterlists, and stops before the @nents-th item if a scatterlist item > >>>>> + * has a marker. > >>>>> + * > >>>>> + * Caller must ensure we don't call this with other virtqueue operations > >>>>> + * at the same time (except where noted). > >>> Hmm so if you want to add in and out, need separate calls? > >>> in_num/out_num would be nicer? > >> > >> If you want to add in and out just use virtqueue_add_buf... > > > > I thought the point of this one is maximum flexibility. > > Maximum flexibility does not include doing everything in one call (the > other way round in fact: you already need to wrap with start/end, hence > doing one or two extra add_sg calls is not important). > > Paolo My point is, we have exactly same number of parameters: in + out instead of nsg + direction, and we get more functionality. -- MST