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Subject: Re: [virtio-comment] [PATCH v2] virtio-net: support distinguishing between partial and full checksum
On Thu, Nov 2, 2023 at 2:50âPM Michael S. Tsirkin <mst@redhat.com> wrote: > > On Thu, Nov 02, 2023 at 12:40:03PM +0800, Jason Wang wrote: > > On Wed, Nov 1, 2023 at 1:37âPM Michael S. Tsirkin <mst@redhat.com> wrote: > > > > > > On Wed, Nov 01, 2023 at 12:16:23PM +0800, Jason Wang wrote: > > > > On Sat, Oct 28, 2023 at 10:36âAM Heng Qi <hengqi@linux.alibaba.com> wrote: > > > > > > > > > > > > > > > > > > > > å 2023/10/27 äå3:39, Michael S. Tsirkin åé: > > > > > > On Thu, Oct 19, 2023 at 02:17:20PM +0800, Heng Qi wrote: > > > > > >> virtio-net works in a virtualized system and is somewhat different from > > > > > >> physical nics. One of the differences is that to save virtio device > > > > > >> resources, rx may receive packets with partial checksum. However, XDP may > > > > > >> cause partially checksummed packets to be dropped. So XDP loading conflicts > > > > > >> with the feature VIRTIO_NET_F_GUEST_CSUM. > > > > > >> > > > > > >> This patch lets the device to supply fully checksummed packets to the driver. > > > > > >> Then XDP can coexist with VIRTIO_NET_F_GUEST_CSUM to enjoy the benefits of > > > > > >> device verification checksum. > > > > > >> > > > > > >> In addition, implementation of some performant devices do not generate > > > > > >> partially checksummed packets, but the standard driver still need to clear > > > > > >> VIRTIO_NET_F_GUEST_CSUM when loading XDP. If these devices enable the > > > > > >> full checksum offloading, then the driver can load XDP without clearing > > > > > >> VIRTIO_NET_F_GUEST_CSUM. > > > > > >> > > > > > >> A new feature bit VIRTIO_NET_F_GUEST_FULL_CSUM is added to solve the above > > > > > >> situation, which provides the driver with configurable receive full checksum > > > > > >> offload. If the offload is enabled, then the device must supply fully > > > > > >> checksummed packets to the driver. > > > > > >> > > > > > >> Use case example: > > > > > >> If VIRTIO_NET_F_GUEST_FULL_CSUM is negotiated and receive full checksum > > > > > >> offload is enabled, after XDP processes a packet with full checksum, the > > > > > >> VIRTIO_NET_HDR_F_DATA_VALID bit is still retained, resulting in the stack > > > > > >> not needing to validate the checksum again. This is useful for guests: > > > > > >> 1. Bring the driver advantages such as cpu savings. > > > > > >> 2. For devices that do not generate partially checksummed packets themselves, > > > > > >> XDP can be loaded in the driver without modifying the hardware behavior. > > > > > >> > > > > > >> Several solutions have been discussed in the previous proposal[1]. > > > > > >> After historical discussion, we have tried the method proposed by Jason[2], > > > > > >> but some complex scenarios and challenges are difficult to deal with. > > > > > >> We now return to the method suggested in [1]. > > > > > >> > > > > > >> [1] https://lists.oasis-open.org/archives/virtio-dev/202305/msg00291.html > > > > > >> [2] https://lore.kernel.org/all/20230628030506.2213-1-hengqi@linux.alibaba.com/ > > > > > >> > > > > > >> Signed-off-by: Heng Qi <hengqi@linux.alibaba.com> > > > > > >> Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com> > > > > > >> --- > > > > > >> v1->v2: > > > > > >> 1. Modify full checksum functionality as a configurable offload > > > > > >> that is initially turned off. @Jason > > > > > >> > > > > > >> device-types/net/description.tex | 54 ++++++++++++++++++++++++++++---- > > > > > >> 1 file changed, 48 insertions(+), 6 deletions(-) > > > > > >> > > > > > >> diff --git a/device-types/net/description.tex b/device-types/net/description.tex > > > > > >> index 76585b0..3c34f27 100644 > > > > > >> --- a/device-types/net/description.tex > > > > > >> +++ b/device-types/net/description.tex > > > > > >> @@ -88,6 +88,8 @@ \subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits > > > > > >> \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through control > > > > > >> channel. > > > > > >> > > > > > >> +\item[VIRTIO_NET_F_GUEST_FULL_CSUM (50)] Driver handles packets with full checksum. > > > > > >> + > > > > > >> \item[VIRTIO_NET_F_HASH_TUNNEL(51)] Device supports inner header hash for encapsulated packets. > > > > > >> > > > > > >> \item[VIRTIO_NET_F_VQ_NOTF_COAL(52)] Device supports virtqueue notification coalescing. > > > > > >> @@ -133,6 +135,7 @@ \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device > > > > > >> \item[VIRTIO_NET_F_GUEST_UFO] Requires VIRTIO_NET_F_GUEST_CSUM. > > > > > >> \item[VIRTIO_NET_F_GUEST_USO4] Requires VIRTIO_NET_F_GUEST_CSUM. > > > > > >> \item[VIRTIO_NET_F_GUEST_USO6] Requires VIRTIO_NET_F_GUEST_CSUM. > > > > > >> +\item[VIRTIO_NET_F_GUEST_FULL_CSUM] Requires VIRTIO_NET_F_GUEST_CSUM and VIRTIO_NET_F_CTRL_GUEST_OFFLOADS. > > > > > >> > > > > > >> \item[VIRTIO_NET_F_HOST_TSO4] Requires VIRTIO_NET_F_CSUM. > > > > > >> \item[VIRTIO_NET_F_HOST_TSO6] Requires VIRTIO_NET_F_CSUM. > > > > > > What about all of these: > > > > > > > > > > > > device-types/net/description.tex:\item[VIRTIO_NET_F_GUEST_TSO4] Requires VIRTIO_NET_F_GUEST_CSUM. > > > > > > device-types/net/description.tex:\item[VIRTIO_NET_F_GUEST_TSO6] Requires VIRTIO_NET_F_GUEST_CSUM. > > > > > > device-types/net/description.tex:\item[VIRTIO_NET_F_GUEST_UFO] Requires VIRTIO_NET_F_GUEST_CSUM. > > > > > > device-types/net/description.tex:\item[VIRTIO_NET_F_GUEST_USO4] Requires VIRTIO_NET_F_GUEST_CSUM. > > > > > > device-types/net/description.tex:\item[VIRTIO_NET_F_GUEST_USO6] Requires VIRTIO_NET_F_GUEST_CSUM. > > > > > > > > > > > > > > > > > > > > > > > > can TSO/UFO/USO work with VIRTIO_NET_F_GUEST_FULL_CSUM as opposed to VIRTIO_NET_F_GUEST_CSUM? > > > > > > > > > > Both GUEST_FULL_CSUM and GUEST_CSUM can work with GUEST_TSO/USO/UFO. > > > > > > > > Yes. For software devices I guess it will have a lot of performance > > > > penalty. So it should be disabled by default anyhow. The idea is to > > > > delay the csum as late as possible. > > > > > > But for hardware it's actually better. > > > > I can't think of a case where it might be better than XDP. > > Of course CHECKSUM_COMPLETE is better than CHECKSUM_PARTIAL if you can > support it: you don't even need to look at csum_start + csum_offset. I may miss something here, if the packet was delivered to the userspace then they don't care. > And the hardware doesn't need to parse L3/L4 headers to implement > CHECKSUM_COMPLETE. Hardware may choose to coalesce packets. > > > > Most userspace doesn't care about the checksum though. > > > > > Maybe we need a flag > > > to say which offloads are expensive? > > > > > > > That exposes some device details which seem not good (e.g we may want > > to do migration among hardware and software). > > > > Thanks > > If you do then things will be less well tuned on one of the migration > ends but then that is by design, isn't it? Ok, so I'm fine to enable it by default. Thanks > > -- > MST >
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