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Subject: Re: [virtio-comment] [PATCH v2] virtio-net: support distinguishing between partial and full checksum
On Fri, 27 Oct 2023 03:39:48 -0400, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 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?
Why is it opposite, I understand that full csum is a subset of it.
>
>
> > @@ -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
It is ok. But as you said, we must change many places that depend on
VIRTIO_NET_F_GUEST_CSUM.
So I more like think VIRTIO_NET_F_GUEST_FULL_CSUM as the subset of VIRTIO_NET_F_GUEST_CSUM.
Then all the cases depended on the VIRTIO_NET_F_GUEST_CSUM can
still work with VIRTIO_NET_F_GUEST_FULL_CSUM & VIRTIO_NET_F_GUEST_CSUM.
So:
- if VIRTIO_NET_F_GUEST_CSUM is not set, VIRTIO_NET_F_GUEST_FULL_CSUM MUST be clear.
>
> - 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
> >
> >
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