From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:46980) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dVYjP-0005w1-SN for qemu-devel@nongnu.org; Thu, 13 Jul 2017 03:40:16 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dVYjK-0000S0-W6 for qemu-devel@nongnu.org; Thu, 13 Jul 2017 03:40:15 -0400 Received: from mga14.intel.com ([192.55.52.115]:12225) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dVYjK-0000Or-Np for qemu-devel@nongnu.org; Thu, 13 Jul 2017 03:40:10 -0400 Message-ID: <5967246B.9030804@intel.com> Date: Thu, 13 Jul 2017 15:42:35 +0800 From: Wei Wang MIME-Version: 1.0 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> In-Reply-To: <20170712163746-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v12 5/8] virtio-balloon: VIRTIO_BALLOON_F_SG List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" 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 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. 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? > - some kind of iterator type would be nicer instead of > maintaining head/prev explicitly Why would we need to iterate 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? 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