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

["Michael S. Tsirkin" <mst@redhat.com>]

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. Maybe we need a flag
to say which offloads are expensive?


> > 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