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Subject: Re: [PATCH v8] virtio-net: correct conditions for devices to validate packet checksum
On Wed, 3 Jan 2024 14:35:02 +0800, Heng Qi <hengqi@linux.alibaba.com> wrote: > There is a historical error in virtio spec: > "If VIRTIO_NET_F_GUEST_CSUM is not negotiated, the device *MUST* set flags > to zero and SHOULD supply a fully checksummed packet to the driver." > > Currently in Linux and virtio-related implementations, the device validates > the packet checksum and sets DATA_VALID regardless of whether > VIRTIO_NET_F_GUEST_CSUM is negotiated. > > Please refer to the following summary and thread[1] for details and reasons: > > Summary > ============= > 1. GUEST_CSUM at virtio spec 0.95 is intended to be compatible with partially > checksummed packets (NEEDS_CSUM <-> CHECKSUM_PARTIAL). So GUEST_CSUM is mapped > to NETIF_F_RXCSUM. > GUEST_CSUM only indicates whether the driver handles partially checksummed packets. > When XDP is loaded, the GUEST_CSUM's offload will be disabled, which means that > packets have NEEDS_CSUM set will be dropped, and packets have DATA_VALID set will > still be received. > > 2. When DATA_VALID was added to Linux in 2011[2] and virtio1.0, it was actually > expected that rx checksum offload (the driver can set CHECKSUM_UNNECESSARY) had > nothing to do with whether GUEST_CSUM was negotiated. But due to an error, below > desctiption was added incorrectly: > "If VIRTIO_NET_F_GUEST_CSUM is not negotiated, the device *MUST* set flags > to zero and SHOULD supply a fully checksummed packet to the driver." > > 3. We now hope to correct this error. Let the setting of DATA_VALID not be > controlled by whether GUEST_CSUM is negotiated, but only controlled by whether > rx checksum offload is enabled on the OS side. The state of this rx checksum > offload is not aware of the device. > > 4. [Optional] NETIF_F_RXCSUM corresponding to rx checksum offload should be > added to dev->hw_features. When the user turns off rx checksum offload through > ethtool -K, neither NEEDS_CSUM nor DATA_VALID should be taken care of, that is, > all packets will be marked as CHECKSUM_NONE. > > Related Link > ============= > [1] https://lists.oasis-open.org/archives/virtio-comment/202312/msg00135.html > [2] 10a8d94a9574 ("virtio_net: introduce VIRTIO_NET_HDR_F_DATA_VALID") > > Fixes: https://github.com/oasis-tcs/virtio-spec/issues/185 > > Suggested-by: Jason Wang <jasowang@redhat.com> > Signed-off-by: Heng Qi <hengqi@linux.alibaba.com> Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com> > --- > v7->v8: > - Drop the new FULLY_CSUM feature and optimize the description. @Jason > > v6->v7: > - FULLY_CSUM no longer depends on GUEST_CSUM. > - The description related to DATA_VALID has nothing to do with > whether GUEST_CSUM is negotiated or not. @Jason > > v5->v6: > - Rewrite and clarify patch description. @Michael > > v4->v5: > - Remove the modification to the GUEST_CSUM. @Jason > - The description of this feature has been reorganized for greater clarity. @Michael > > v3->v4: > - Streamline some repetitive descriptions. @Jason > - Add how features should work, when to be enabled, and overhead. @Jason @Michael > > v2->v3: > - Add a section named "Driver Handles Fully Checksummed Packets" > and more descriptions. @Michael > > v1->v2: > - Modify full checksum functionality as a configurable offload > that is initially turned off. @Jason > > device-types/net/description.tex | 14 +++----------- > 1 file changed, 3 insertions(+), 11 deletions(-) > > diff --git a/device-types/net/description.tex b/device-types/net/description.tex > index aff5e08..14978c0 100644 > --- a/device-types/net/description.tex > +++ b/device-types/net/description.tex > @@ -723,8 +723,7 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network > \field{num_buffers} is one, then the entire packet will be > contained within this buffer, immediately following the struct > virtio_net_hdr. > -\item If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the > - VIRTIO_NET_HDR_F_DATA_VALID bit in \field{flags} can be > +\item The VIRTIO_NET_HDR_F_DATA_VALID bit in \field{flags} can be > set: if so, device has validated the packet checksum. > In case of multiple encapsulated protocols, one level of checksums > has been validated. > @@ -783,9 +782,8 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network > packet together, such that at least \field{num_buffers} are > observed by driver as used. > > -If VIRTIO_NET_F_GUEST_CSUM is not negotiated, the device MUST set > -\field{flags} to zero and SHOULD supply a fully checksummed > -packet to the driver. > +If VIRTIO_NET_F_GUEST_CSUM is not negotiated, the device SHOULD supply > +a fully checksummed packet to the driver. > > If VIRTIO_NET_F_GUEST_TSO4 is not negotiated, the device MUST NOT set > \field{gso_type} to VIRTIO_NET_HDR_GSO_TCPV4. > @@ -842,12 +840,6 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network > not less than the length of the headers, including the transport > header. > > -If the VIRTIO_NET_F_GUEST_CSUM feature has been negotiated, the > -device MAY set the VIRTIO_NET_HDR_F_DATA_VALID bit in > -\field{flags}, if so, the device MUST validate the packet > -checksum (in case of multiple encapsulated protocols, one level > -of checksums is validated). > - > \drivernormative{\paragraph}{Processing of Incoming > Packets}{Device Types / Network Device / Device Operation / > Processing of Incoming Packets} > -- > 1.8.3.1 >
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