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Subject: Re: [virtio-comment] [PATCH v2] virtio-net: support distinguishing between partial and full checksum
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? > @@ -390,6 +393,13 @@ \subsection{Device Initialization}\label{sec:Device Types / Network Device / Dev > \ref{sec:Device Types / Network Device / Device Operation / > Processing of Incoming Packets}~\nameref{sec:Device Types / > Network Device / Device Operation / Processing of Incoming Packets} below. > + > +\item The VIRTIO_NET_F_GUEST_FULL_CSUM feature indicates that the driver handles > + packets with full checksum and does not handle packets with partial checksum, So we need to change definition of VIRTIO_NET_F_GUEST_CSUM then. Also this is not exactly right. As defined driver must be able to handle partial checksum too. How about this: - change definition above to just "Driver handles packets with full checksum." - if VIRTIO_NET_F_GUEST_FULL_CSUM is set but VIRTIO_NET_F_GUEST_CSUM is clear driver requires full checksum - if VIRTIO_NET_F_GUEST_FULL_CSUM is clear but VIRTIO_NET_F_GUEST_CSUM is set driver supports partial checksum - if VIRTIO_NET_F_GUEST_FULL_CSUM and VIRTIO_NET_F_GUEST_CSUM are set then the behavior is as you describe: VIRTIO_NET_F_GUEST_CSUM takes preference, but you can disable it with VIRTIO_NET_F_CTRL_GUEST_OFFLOADS if that is supported. > + which means that partially checksummed packets can not be received by the driver > + and the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in \field{flags} can not be set by the device. > + The driver can send the VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET command to control > + the receive full checksum offload state. I don't like it that we are coming with "offload state" here that isn't explained anywhere. Let's try to make this offload more like other existing offloads. I find it is also confusing to say "receive full checksum offload has been enabled". It is more that driver handles packets with full checksum. > \end{enumerate} > > A truly minimal driver would only accept VIRTIO_NET_F_MAC and ignore > @@ -720,7 +730,8 @@ \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 > +\item If the VIRTIO_NET_F_GUEST_CSUM feature (regardless of whether > + VIRTIO_NET_F_GUEST_FULL_CSUM was negotiated) was negotiated, 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 > @@ -744,7 +755,8 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network > number of coalesced TCP segments in \field{csum_start} field and > number of duplicated ACK segments in \field{csum_offset} field > and sets bit VIRTIO_NET_HDR_F_RSC_INFO in \field{flags}. > -\item If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the > +\item If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated but the > + VIRTIO_NET_F_GUEST_FULL_CSUM feature was not 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 > @@ -802,8 +814,9 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network > device MUST set the VIRTIO_NET_HDR_GSO_ECN bit in > \field{gso_type}. > > -If the VIRTIO_NET_F_GUEST_CSUM feature has been negotiated, the > -device MAY set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in > +If the VIRTIO_NET_F_GUEST_CSUM feature has been negotiated but > +the VIRTIO_NET_F_GUEST_FULL_CSUM feature has not been negotiated, > +the device MAY set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in > \field{flags}, if so: > \begin{enumerate} > \item the device MUST validate the packet checksum at > @@ -819,11 +832,38 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network > fully checksummed packet; > \end{enumerate} > > +The driver MUST NOT enable receive full checksum offload for which > +VIRTIO_NET_F_GUEST_FULL_CSUM has not been negotiated. > + > +Initially (before the device receives any VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET > +command carrying the VIRTIO_NET_F_GUEST_FULL_CSUM feature parameter) receive > +full checksum offload MUST be disabled. > + > +Upon the device reset, the device MUST disable receive full checksum offload. > + > +If VIRTIO_NET_F_GUEST_FULL_CSUM has been negotiated and receive full checksum > +offload has not been enabled, the device MUST NOT perform any of the > +functionality provided by VIRTIO_NET_F_GUEST_FULL_CSUM. > + > +If receive full checksum offload has been enabled, the device MUST NOT set > +the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in \field{flags} and MUST supply a > +fully checksummed packet to the driver. > + > +If a partially checksummed packet is received by the device, the device MUST > +calculate full checksum for the packet and then supply it to the driver > +\ref{sec:Device Types / Network Device / Device Operation / Packet Transmission}. > + > If none of the VIRTIO_NET_F_GUEST_TSO4, TSO6, UFO, USO4 or USO6 options have > been negotiated, the device MUST set \field{gso_type} to > VIRTIO_NET_HDR_GSO_NONE. > > -If \field{gso_type} differs from VIRTIO_NET_HDR_GSO_NONE, then > +If receive full checksum offload has been enabled and \field{gso_type} > +differs from VIRTIO_NET_HDR_GSO_NONE, then the device MUST NOT set > +the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in \field{flags} and MUST calculate > +full checksum for the packet and then supply it to the driver. > + > +If the VIRTIO_NET_F_GUEST_FULL_CSUM feature has not been negotiated and > +\field{gso_type} differs from VIRTIO_NET_HDR_GSO_NONE, then > the device MUST also set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in > \field{flags} MUST set \field{gso_size} to indicate the desired MSS. > If VIRTIO_NET_F_RSC_EXT was negotiated, the device MUST also > @@ -839,7 +879,8 @@ \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 > +If the VIRTIO_NET_F_GUEST_CSUM feature (regardless of whether > +VIRTIO_NET_F_GUEST_FULL_CSUM has been negotiated) 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 > @@ -1624,6 +1665,7 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi > #define VIRTIO_NET_F_GUEST_TSO6 8 > #define VIRTIO_NET_F_GUEST_ECN 9 > #define VIRTIO_NET_F_GUEST_UFO 10 > +#define VIRTIO_NET_F_GUEST_FULL_CSUM 50 > #define VIRTIO_NET_F_GUEST_USO4 54 > #define VIRTIO_NET_F_GUEST_USO6 55 > > -- > 2.19.1.6.gb485710b > > > 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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