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Subject: Re: [virtio-comment] Re: [virtio-dev] Re: [Proposal] Relationship between XDP and rx-csum in virtio-nety
On Wed, May 24, 2023 at 12:09:22PM +0800, Jason Wang wrote: > On Tue, May 23, 2023 at 9:51âPM Heng Qi <hengqi@linux.alibaba.com> wrote: > > > > On Tue, May 23, 2023 at 09:30:28AM -0400, Michael S. Tsirkin wrote: > > > On Tue, May 23, 2023 at 05:18:20PM +0800, Heng Qi wrote: > > > > On Tue, May 23, 2023 at 03:15:37AM -0400, Michael S. Tsirkin wrote: > > > > > On Tue, May 23, 2023 at 10:41:18AM +0800, Heng Qi wrote: > > > > > > On Mon, May 22, 2023 at 03:10:05PM -0400, Michael S. Tsirkin wrote: > > > > > > > On Mon, May 22, 2023 at 08:12:00PM +0800, Heng Qi wrote: > > > > > > > > 1) Add a feature bit to the virtio specification to tell the sender that a fully > > > > > > > > csumed packet must be sent. > > > > > > > > > > > > > > Who is the sender in this picture? The driver? > > > > > > > > > > > > The device or the driver. > > > > > > > > > > > > When the device is hw, the sender is more likely to be a device. > > > > > > When the device is sw, the sender can be a device or a driver. > > > > > > > > > > > > But in general, this feature is inclined to constrain the behavior of the device and > > > > > > the driver from the receiving side. > > > > > > > > > > Based on above I am guessing you are talking about driver getting > > > > > packets from device, I wish you used terms from virtio spec. > > > > > > > > Yes, I'm going to use the terminology of the virtio spec. > > > > > > > > > > > > > > > For example: > > > > > > VIRTIO_NET_F_UNNECESSARY_CSUM : The driver tells the device that you must send me a fully csumed packet. > > > > > > > > > > > > Then the specific implementation can be > > > > > > > > > > > > (1) the sender sends a fully csumed packet; > > > > > > (2) the receiver receives a CHECKSUM_PARTIAL packet, and the device helps calculate the fully csum > > > > > > (because the two parties in the communication are located on the same host, the packet is trusted.). > > > > > > > > > > > > In summary, if VIRTIO_NET_F_UNNECESSARY_CSUM is negotiated, the driver will no longer receive any packets marked CHECKSUM_PARTIAL. > > > > > > > > > > > > Thanks. > > > > > > > > > > This is what clearing VIRTIO_NET_F_GUEST_CSUM does. > > > > > > > > Yes, but with VIRTIO_NET_F_GUEST_CSUM cleared, although the device can > > > > receive a fully checksummed packet, we can no longer enjoy > > > > the device's ability to validate the packet checksum. That is, the value > > > > of \field{flags} in the virtio_net_hdr structure is set to 0, which means > > > > that the packet received by the driver will not be marked as > > > > VIRTIO_NET_HDR_F_DATA_VALID. > > > > > > > > So, we need a feature bit (let's say VIRTIO_NET_F_UNNECESSARY_CSUM). > > > > If VIRTIO_NET_F_UNNECESSARY_CSUM is negotiated, the device must give the > > > > driver a fully checksummed packet, and the packet is validated by the > > > > device with \field{flags} set to VIRTIO_NET_HDR_F_DATA_VALID. > > > > > > > > > > > > > > I feel you are trying to say that clearing VIRTIO_NET_F_GUEST_CSUM > > > > > disables all offloads but you want to keep some of them? > > > > > > > > > > > > > No, what I mean is that a feature VIRTIO_NET_F_UNNECESSARY_CSUM is needed > > > > in addition to VIRTIO_NET_F_GUEST_CSUM, if both features are negotiated, > > > > then the driver may always receive packets marked as > > > > VIRTIO_NET_HDR_F_DATA_VALID, which means that we can now load XDP at the > > > > same time. > > > > > > Makes no sense to me. VIRTIO_NET_F_GUEST_CSUM set already allows > > > VIRTIO_NET_HDR_F_DATA_VALID: > > > > We need to focus on what happens when the XDP program is loaded: > > > > The current virtio-net needs to turn off the VIRTIO_NET_F_GUEST_CSUM > > feature when loading XDP. The main reason for doing this is because > > VIRTIO_NET_F_GUEST_CSUM allows to receive packets marked as > > VIRTIO_NET_HDR_F_NEEDS_CSUM. Such packets are not compatible with > > XDP programs, because we cannot guarantee that the csum_{start, offset} > > fields are correct after XDP modifies the packets. > > We can try to fix or workaround this. A rough idea is for example by > using a flow dissector? How can we fix this with a flow dissector? Could you please explain in detail? > Anyhow the GSO of virtio-net was marked as > dodgy which means the csum_start/offset needs to be validated again in > the xmit path. Or we can monitor if the packet is modified or not, if > not, we don't need to zero vnet header? Yes, this is a way, and we've thought about this solution. A relatively simple solution is to add a new flag to the XDP program (through XDP core layer), which indicates that the XDP program is a read-only XDP program (ie, does not modify the packet). Then to load such an XDP program we no longer need to filter out the VIRTIO_NET_F_GUEST_CSUM feature. But why we didn't propose this solution in the 'Proposal', because it seems that only virtio-net has encountered related problems. Virtual network cards like veth, even if they receive partial csumed packets (corresponding to virtio-net's packets marked as VIRTIO_NET_HDR_F_NEEDS_CSUM), they still open NETIF_F_RXCSUM (corresponding to virtio-net's VIRTIO_NET_F_GUEST_CSUM). So there is another trade-off: the current virtnet_xdp_set() no longer filters out VIRTIO_NET_F_GUEST_CSUM. The reasons I think are the following: 1. Referring to vethâs way, the XDP program is more of a monitoring/firewall, and may not modify data packets. 2. We can still use XDP for packets marked as VIRTIO_NET_HDR_F_DATA_VALID. 3. Packets converted from xdp_{buff, frame} may not be ip_forwarded (because ip_forward sysctl is disabled by default) Thanks. > > Thanks > > > > > So in order for the driver to continue to enjoy the device's ability to > > validate packets while XDP is loaded, we need a new feature to tell the > > device not to deliver packets marked as VIRTIO_NET_HDR_F_NEEDS_CSUM (in > > other words, the device can only deliver packets marked as > > VIRTIO_NET_HDR_F_DATA_VALID). > > > > Thanks. > > > > > \item If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the > > > VIRTIO_NET_HDR_F_NEEDS_CSUM bit in \field{flags} can be > > > set: if so, the packet checksum at offset \field{csum_offset} > > > from \field{csum_start} and any preceding checksums > > > have been validated. The checksum on the packet is incomplete and > > > if bit VIRTIO_NET_HDR_F_RSC_INFO is not set in \field{flags}, > > > then \field{csum_start} and \field{csum_offset} indicate how to calculate it > > > (see Packet Transmission point 1). > > > > > > Did you maybe mean if either feature is negotiated? > > > > > > > > > > > > > > Again please use virtio terminology not Linux. to help you out, > > > > > in current linux, VIRTIO_NET_HDR_F_NEEDS_CSUM and VIRTIO_NET_HDR_F_DATA_VALID > > > > > will set CHECKSUM_PARTIAL and CHECKSUM_UNNECESSARY respectively. > > > > > > > > > > > > > Sure. Will do as you suggested. > > > > > > > > Thanks. > > > > > > > > > > > > > > -- > > > > > MST > > > > > > > > > > > > > > > --------------------------------------------------------------------- > > > > > To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org > > > > > For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org > > > > > > > > > This publicly archived list offers a means to provide input to the > > > OASIS Virtual I/O Device (VIRTIO) TC. > > > > > > In order to verify user consent to the Feedback License terms and > > > to minimize spam in the list archive, subscription is required > > > before posting. > > > > > > Subscribe: virtio-comment-subscribe@lists.oasis-open.org > > > Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org > > > List help: virtio-comment-help@lists.oasis-open.org > > > List archive: https://lists.oasis-open.org/archives/virtio-comment/ > > > Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf > > > List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lists > > > Committee: https://www.oasis-open.org/committees/virtio/ > > > Join OASIS: https://www.oasis-open.org/join/ > >
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