From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id BF0EF986584 for ; Fri, 9 Sep 2022 12:45:05 +0000 (UTC) Message-ID: <1662727108.301059-1-xuanzhuo@linux.alibaba.com> Date: Fri, 9 Sep 2022 20:38:28 +0800 From: Xuan Zhuo References: <1660642495-104002-1-git-send-email-hengqi@linux.alibaba.com>

<20220904162337-mutt-send-email-mst@kernel.org> <9843a9ea-07b9-7966-efa0-debf0467902a@linux.alibaba.com> <20220909044129-mutt-send-email-mst@kernel.org> In-Reply-To: <20220909044129-mutt-send-email-mst@kernel.org> Subject: [virtio-dev] Re: [PATCH v7] virtio_net: support split header Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable To: "Michael S. Tsirkin" Cc: virtio-dev@lists.oasis-open.org, Jason Wang , kangjie.xu@linux.alibaba.com, Heng Qi List-ID: On Fri, 9 Sep 2022 07:15:02 -0400, "Michael S. Tsirkin" wr= ote: > On Fri, Sep 09, 2022 at 03:41:54PM +0800, Heng Qi wrote: > > > > > > =E5=9C=A8 2022/9/5 =E4=B8=8A=E5=8D=884:27, Michael S. Tsirkin =E5=86=99= =E9=81=93: > > > On Fri, Sep 02, 2022 at 03:36:25PM +0800, Heng Qi wrote: > > > > We need to clarify that the purpose of header splitting is to make = all payloads > > > > can be independently in a page, which is beneficial for the zerocop= y > > > > implemented by the upper layer. > > > absolutely, pls add motivation. > > > > > > > If the driver does not enforce that the buffers submitted to the re= ceiveq MUST > > > > be composed of at least two descriptors, then header splitting will= become meaningless, > > > > or the VIRTIO_NET_F_SPLIT_TRANSPORT_HEADER feature should not be ne= gotiated at this time. > > > > > > > > > > > > Thanks. > > > > > > > > > > > > > > This seems very narrow and unecessarily wasteful of descriptors. > > > What is wrong in this: > > > > > >

...... > > > > > > seems to achieve the goal of data in a separate page without > > > using extra descriptors. > > > > > > thus my proposal to replace the requirement of a separate > > > descriptor with an offset of data from beginning of > > > buffer that driver sets. > > > > > > > > We have carefully considered your suggestion. > > > > We refer to spec v7 and earlier as scheme A for short. Review scheme A > > below: > > > > | receive buffer | > > > > | 0th descriptor | 1th descriptor | > > > > | virtnet hdr | mac | ip hdr | tcp hdr|<-- hold -->| payload | > > > > We use a buffer plus a separate page when allocating the receive > > > > buffer. In this way, we can ensure that all payloads can be > > > > independently in a page, which is very beneficial for the zerocopy > > > > implemented by the upper layer. > > > > scheme A better solves the problem of headroom, tailroom and memory was= te, > > but as you said, this solution relies on descriptor chain. > > > > Our rethinking approach is no longer based on or using descriptor chain= . > > > > We refer to your proposed offset-based scheme as scheme B: > > > > As you suggested, scheme B gives the device a buffer, using offset to > > indicate where to place the payload like this: > > > >

...... > > > > But how to apply for this buffer? Since we want the payload to be place= d on > > a separate page, the method we consider is to directly apply to the dri= ver > > for two pages of contiguous memory. > > > > Then the beginning of this contiguous memory is used to store the headr= oom, > > and the contiguous memory after the headroom is directly handed over to= the > > device. similar to the following: > > > > <------------------------------------------ receive buffer(2 pages) > > -----------------------------------------> > > > > <<---------------------------------- first page > > -----------------------------------><---- second page ------>> > > > > < > header>....> > > > > Based on your previous suggestion, we also considered another new schem= e C. > > > > This scheme is implemented based on mergeable buffer, filling a separat= e > > page each time. > > > > If the split header is negotiated and the packet can be successfully sp= lit > > by the device, the device needs to find at least two buffers, namely tw= o > > pages, one for the virtio-net header and transport header, and the othe= r for > > the data payload. Like the following: > > > > | receive buffer1(page) | receive buffer2 (page) | > > > > | virtnet hdr | mac | ip hdr | tcp hdr|<-- hold -->| payload | > > > > At the same time, if XDP is considered, then the device needs to add > > headroom at the beginning of receive buffer1 when receiving packets, so= that > > the driver can process programs similar to XDP. In order to solve this > > problem, can scheme C introduce an offset, which requires the device to > > write data from the offset position to receive buffer1, like the follow= ing: > > > > | receive buffer (page) | receive buffer (page) | > > > > | <-- offset(hold) --> | virtnet hdr | mac | ip hdr | tcp hdr|<-- hold = -->| > > payload | > > And in fact, B and C both use an offset now, right? B: offset is used to get the position to place the payload. C: The offset is used to reserve some space for the device, which the drive= r can use as headroom. In order to make the payload page-aligned, we can only hand over the ent= ire page to the device, so we cannot reserve some headroom in advance. > > > Then we simply compare the advantages and disadvantages of scheme A(spe= c > > v7), scheme B (offset buffer(2 pages)) and scheme C (based on mergeable > > buffer): > > > > 1. desc chain: > > > > - A depends on desciptor chain; - B, C do not depend on desciptor chain= . > > > > 2. page alloc > > > > - B fills two consecutive pages, which causes a great waste of memory f= or > > small packages such as arp; - C fills a single page, slightly better th= an B. > > > > 3. Memory waste: > > > > - The memory waste of scheme A is mainly the 0th descriptor that is ski= pped > > by the device; > > there's also the cost of the indirect buffer since that is used when > there is a chain. Yes > > > - When scheme B and scheme C successfully split the header, > > there is a huge waste of the first page, but the first page can be quic= kly > > released by copying. > > > > 4. headroom > > > > - The headrooms of plan A and plan B are reserved; - Scheme C requires = the > > driver to set off to let the device skip off when using receive buffer1= . > > > > 5. tailroom > > > > - When splitting the header, skb usually needs to store each independen= t > > page in the non-linear data area based on shinfo. - The tailroom of sch= eme A > > is reserved by itself; - Scheme B requires the driver to set the reserv= ed > > padding area for the first receive buffer(2 pages) to use shinfo when t= he > > split header is not successfully executed; - Scheme C requires the driv= er to > > set max_len for the first receive buffer(page). > > > > > > Which plan do you prefer? > > I think either both B and C depending on the mergeable buffers flag, > or just one of these two. If I understand correctly, B does not depend on mergeable, while C must dep= end on mergeable. Thanks. > > > --- > > > > Thanks. > --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org