From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753190AbdGMUTb (ORCPT ); Thu, 13 Jul 2017 16:19:31 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36170 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753046AbdGMUTa (ORCPT ); Thu, 13 Jul 2017 16:19:30 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 2023780C26 Authentication-Results: ext-mx02.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx02.extmail.prod.ext.phx2.redhat.com; spf=pass smtp.mailfrom=mst@redhat.com DKIM-Filter: OpenDKIM Filter v2.11.0 mx1.redhat.com 2023780C26 Date: Thu, 13 Jul 2017 23:19:22 +0300 From: "Michael S. Tsirkin" To: Wei Wang Cc: linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, linux-mm@kvack.org, david@redhat.com, cornelia.huck@de.ibm.com, akpm@linux-foundation.org, mgorman@techsingularity.net, aarcange@redhat.com, amit.shah@redhat.com, pbonzini@redhat.com, liliang.opensource@gmail.com, virtio-dev@lists.oasis-open.org, yang.zhang.wz@gmail.com, quan.xu@aliyun.com Subject: Re: [PATCH v12 5/8] virtio-balloon: VIRTIO_BALLOON_F_SG Message-ID: <20170713210819-mutt-send-email-mst@kernel.org> References: <1499863221-16206-1-git-send-email-wei.w.wang@intel.com> <1499863221-16206-6-git-send-email-wei.w.wang@intel.com> <20170712160129-mutt-send-email-mst@kernel.org> <5966241C.9060503@intel.com> <20170712163746-mutt-send-email-mst@kernel.org> <5967246B.9030804@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5967246B.9030804@intel.com> X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.26]); Thu, 13 Jul 2017 20:19:29 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 13, 2017 at 03:42:35PM +0800, Wei Wang wrote: > On 07/12/2017 09:56 PM, Michael S. Tsirkin wrote: > > > > So the way I see it, there are several issues: > > > > - internal wait - forces multiple APIs like kick/kick_sync > > note how kick_sync can fail but your code never checks return code > > - need to re-write the last descriptor - might not work > > for alternative layouts which always expose descriptors > > immediately > > Probably it wasn't clear. Please let me explain the two functions here: > > 1) virtqueue_add_chain_desc(vq, head_id, prev_id,..): > grabs a desc from the vq and inserts it to the chain tail (which is indexed > by > prev_id, probably better to call it tail_id). Then, the new added desc > becomes > the tail (i.e. the last desc). The _F_NEXT flag is cleared for each desc > when it's > added to the chain, and set when another desc comes to follow later. And this only works if there are multiple rings like avail + descriptor ring. It won't work e.g. with the proposed new layout where writing out a descriptor exposes it immediately. > 2) virtqueue_add_chain(vq, head_id,..): expose the chain to the other end. > > So, if people want to add a desc and immediately expose it to the other end, > i.e. build a single desc chain, they can just add and expose: > > virtqueue_add_chain_desc(..); > virtqueue_add_chain(..,head_id); > > Would you see any issues here? The way the new APIs poll used ring internally. > > > - some kind of iterator type would be nicer instead of > > maintaining head/prev explicitly > > Why would we need to iterate the chain? In your patches prev/tail are iterators - they keep track of where you are in the chain. > I think it would be simpler to use > a wrapper struct: > > struct virtqueue_desc_chain { > unsigned int head; // head desc id of the chain > unsigned int tail; // tail desc id of the chain > } > > The new desc will be put to desc[tail].next, and we don't need to walk > from the head desc[head].next when inserting a new desc to the chain, right? > > > > > > As for the use, it would be better to do > > > > if (!add_next(vq, ...)) { > > add_last(vq, ...) > > kick > > wait > > } > > "!add_next(vq, ...)" means that the vq is full? No - it means there's only 1 entry left for the last descriptor. > If so, what would add_last() > do then? > > > Using VIRTQUEUE_DESC_ID_INIT seems to avoid a branch in the driver, but > > in fact it merely puts the branch in the virtio code. > > > > Actually it wasn't intended to improve performance. It is used to indicate > the "init" state > of the chain. So, when virtqueue_add_chain_desc(, head_id,..) finds head > id=INIT, it will > assign the grabbed desc id to &head_id. In some sense, it is equivalent to > add_first(). > > Do you have a different opinion here? > > Best, > Wei > It is but let's make it explicit here - an API function is better than a special value. -- MST