Re: [virtio-comment] [PATCH v2] virtio-net: support distinguishing between partial and full checksum

[Jason Wang <jasowang@redhat.com>]

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.

> Their important difference is that if GUEST_CSUM is negotiated, the
> driver can handle partial checksum.
>
> >
> >
> >> @@ -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.
>
> Jason wanted this feature to be enabled only when XDP is loading,
> and this is the context in which this patch was proposed.
>
> How do you pay attention to this?

I don't see any conflict, or anything I miss?

Thanks