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Subject: Re: [virtio-comment] [PATCH v2] virtio-net: support distinguishing between partial and full checksum
On Thu, Nov 02, 2023 at 01:30:22PM +0800, Jason Wang wrote:
> On Wed, Nov 1, 2023 at 12:59âPM Heng Qi <hengqi@linux.alibaba.com> wrote:
> >
> >
> >
> > å 2023/11/1 äå12:16, Jason Wang åé:
> > > 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.
> >
> > Yes. I totally agree.
> >
> > >
> > >> 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?
> >
> > Yes, our request was met.
> >
> > If GUEST_FULL_CSUM and GUEST_CSUM are independent,
> > that is, GUEST_FULL_CSUM can be successfully validated without GUEST_CSUM.
> > Then we need to re-describe most of the existing behavior of GUEST_CSUM
> > for FULL_CSUM
> > in the spec, this part is overlapping. Moreover, the relationship
> > between FULL_CSUM
> > and GUEST_CSUM also needs to be processed in the full text.
> >
> > So I think it seems clearer to constrain the behavior of GUEST_CSUM by
> > treating FULL_CSUM as a subset of GUEST_CSUM.
>
> This seems to work.
>
> Thanks
I feel the changes here are minor and worth making for purposes
of making the features orthogonal.
Further, I feel it's ok to default to offloads off for some features.
However, it is not at all clear from this patch when should driver
enable the offload and when it should not.
The fact that it needs a command to be enabled does not mean much:
drivers are quite capable to invoke VIRTIO_NET_CTRL_GUEST_OFFLOADS
at startup and in fact some do.
So when should driver enable this as opposed to calculating the checksum
in software?
> >
> > For example we don't need to make the following changes:
> > \item[VIRTIO_NET_F_GUEST_TSO4] Requires VIRTIO_NET_F_GUEST_CSUM or
> > VIRTIO_NET_F_GUEST_FULL_CSUM.
> > \item[VIRTIO_NET_F_GUEST_TSO6] Requires VIRTIO_NET_F_GUEST_CSUM or
> > VIRTIO_NET_F_GUEST_FULL_CSUM.
> > \item[VIRTIO_NET_F_GUEST_UFO] Requires VIRTIO_NET_F_GUEST_CSUM or
> > VIRTIO_NET_F_GUEST_FULL_CSUM.
> > \item[VIRTIO_NET_F_GUEST_USO4] Requires VIRTIO_NET_F_GUEST_CSUM or
> > VIRTIO_NET_F_GUEST_FULL_CSUM.
> > \item[VIRTIO_NET_F_GUEST_USO6] Requires VIRTIO_NET_F_GUEST_CSUM or
> > VIRTIO_NET_F_GUEST_FULL_CSUM.
> >
> > Thanks!
> >
> > >
> > > Thanks
> >
> >
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