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Subject: Re: [virtio-dev] Re: [PATCH v7] virtio_net: support split header
On Mon, Sep 05, 2022 at 05:43:27PM +0800, Xuan Zhuo wrote: > On Mon, 5 Sep 2022 16:37:57 +0800, Heng Qi <hengqi@linux.alibaba.com> wrote: > > > > å 2022/9/5 äå3:52, Xuan Zhuo åé: > > > On Sun, 4 Sep 2022 16:31:59 -0400, "Michael S. Tsirkin"<mst@redhat.com> wrote: > > >> On Fri, Sep 02, 2022 at 04:58:16PM +0800, Heng Qi wrote: > > >>> When VIRTIO_NET_F_SPLIT_TRANSPORT_HEADER is negotiated, > > >>> the driver requires that the buffers submitted to receiveq > > >>> MUST be composed of at least two descriptors, > > >>> which means that each buffer the device gets is a descriptor chain, > > >>> even if the device does not split the header for some packets. > > >>> > > >>> To store packet in the descriptor chain without header splitting > > >>> by the device, the device MUST start with the first descriptor of > > >>> the descriptor chain to store the packet, and MUST NOT set the > > >>> VIRTIO_NET_HDR_F_SPLIT_TRANSPORT_HEADER bit in \field{flags}. > > >>> > > >>> Thanks. > > >> Descriptor chains will hurt performance badly. > > > I understand the reasons for the performance impact here are: > > > 1. Two buffers are used > > > 2. One buffer occupies two descs > > > > > > This is the same as my understanding in the case of mergeable. We also need to > > > pack the packets into two buffers, and a packet will eventually occupy two > > > descs. > > > > > > > > >> How about simply making this feature depend on mergeable buffers? > > >> Then we have a separate buffer for the header and > > >> this works cleanly. > > > > > > Under mergeable, each buffer is independent, and the split header requires two > > > unequal descs. > > > > > > If we implement it based on mergeable, then consider the scenario of tcp > > > zerocopy, when we fill receive vq, each buffer is an separate page, and if we use an > > > separate buffer to save the header, then this is a waste, we may > > > have to copy at the driver layer. > > > > > > @Qi Do you think there will be other problems with this approach? > > > > > > Thanks. > > > > > When we think about specs, we shouldn't be too distracted by the implementation. > > > > But when we did think about this, suppose the driver fills by page based on > > mergeable mode. in order to use the xdp program, the driver usually takes > > the beginning of a single page as the headroom, and fills the rest of the page > > into the virtqueue. Therefore, the empty buffer obtained by the > > device is always smaller than a page when we implement split header > > based on this mode, that is, the data load finally obtained by the driver > > is offset from the beginning of the page. This does not enjoy the benefits of zero copy. > > > > At the same time, since the header is always only more than 100 bytes, > > the page occupied by the header is a waste of the buffer. > > > Yeah that reminds me that merge doesn't feel like it handles this very well. > > The essence is that the two buffers used by the split header are different. > > Desc Chain is used to bind a small buffer desc and a page desc. I didn't think > of a better way to deal with this problem. > > Thanks. > I sent some suggestions avoiding use of descriptors completely, using offsets instead. take a look. > > > > Thanks. > > > > > > > > > > > > > > > > --------------------------------------------------------------------- > To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org > For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org
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