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Subject: Re: [virtio-comment] Re: [PATCH v4 1/2] virtio-net: update description for VIRTIO_NET_F_GUEST_CSUM.
On Wed, Dec 6, 2023 at 4:08âPM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>
> On Wed, 6 Dec 2023 12:36:53 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > On Wed, Dec 6, 2023 at 10:21âAM Heng Qi <hengqi@linux.alibaba.com> wrote:
> > >
> > >
> > >
> > > å 2023/12/5 äå10:45, Michael S. Tsirkin åé:
> > > > On Tue, Dec 05, 2023 at 10:18:32PM +0800, Heng Qi wrote:
> > > >>
> > > >> å 2023/12/5 äå11:52, Jason Wang åé:
> > > >>> On Mon, Dec 4, 2023 at 5:34âPM Heng Qi <hengqi@linux.alibaba.com> wrote:
> > > >>>>
> > > >>>> å 2023/12/4 äå5:05, Michael S. Tsirkin åé:
> > > >>>>> On Mon, Dec 04, 2023 at 04:59:49PM +0800, Jason Wang wrote:
> > > >>>>>> On Mon, Dec 4, 2023 at 4:53âPM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > >>>>>>> On Mon, Dec 04, 2023 at 04:49:46PM +0800, Jason Wang wrote:
> > > >>>>>>>> On Mon, Dec 4, 2023 at 3:37âPM Heng Qi <hengqi@linux.alibaba.com> wrote:
> > > >>>>>>>>> å 2023/12/4 äå3:18, Jason Wang åé:
> > > >>>>>>>>>> On Fri, Dec 1, 2023 at 3:16âPM Heng Qi <hengqi@linux.alibaba.com> wrote:
> > > >>>>>>>>>>> å 2023/12/1 äå3:05, Jason Wang åé:
> > > >>>>>>>>>>>> On Fri, Dec 1, 2023 at 2:30âPM Heng Qi <hengqi@linux.alibaba.com> wrote:
> > > >>>>>>>>>>>>> å 2023/12/1 äå2:24, Heng Qi åé:
> > > >>>>>>>>>>>>>> å 2023/12/1 äå1:18, Jason Wang åé:
> > > >>>>>>>>>>>>>>> On Wed, Nov 29, 2023 at 4:23âPM Heng Qi <hengqi@linux.alibaba.com>
> > > >>>>>>>>>>>>>>> wrote:
> > > >>>>>>>>>>>>>>>> å 2023/11/29 äå4:00, Jason Wang åé:
> > > >>>>>>>>>>>>>>>>> On Tue, Nov 28, 2023 at 4:08âPM Heng Qi <hengqi@linux.alibaba.com>
> > > >>>>>>>>>>>>>>>>> wrote:
> > > >>>>>>>>>>>>>>>>>> To prevent readers from misunderstanding that the driver can
> > > >>>>>>>>>>>>>>>>>> only handles packets with partial checksum when
> > > >>>>>>>>>>>>>>>>>> VIRTIO_NET_F_GUEST_CSUM is negotiated, we update the description.
> > > >>>>>>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>>>>>> Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
> > > >>>>>>>>>>>>>>>>>> ---
> > > >>>>>>>>>>>>>>>>>> device-types/net/description.tex | 2 +-
> > > >>>>>>>>>>>>>>>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
> > > >>>>>>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>>>>>> diff --git a/device-types/net/description.tex
> > > >>>>>>>>>>>>>>>>>> b/device-types/net/description.tex
> > > >>>>>>>>>>>>>>>>>> index aff5e08..529f470 100644
> > > >>>>>>>>>>>>>>>>>> --- a/device-types/net/description.tex
> > > >>>>>>>>>>>>>>>>>> +++ b/device-types/net/description.tex
> > > >>>>>>>>>>>>>>>>>> @@ -38,7 +38,7 @@ \subsection{Feature bits}\label{sec:Device Types
> > > >>>>>>>>>>>>>>>>>> / Network Device / Feature bits
> > > >>>>>>>>>>>>>>>>>> \begin{description}
> > > >>>>>>>>>>>>>>>>>> \item[VIRTIO_NET_F_CSUM (0)] Device handles packets with
> > > >>>>>>>>>>>>>>>>>> partial checksum offload.
> > > >>>>>>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>>>>>> -\item[VIRTIO_NET_F_GUEST_CSUM (1)] Driver handles packets with
> > > >>>>>>>>>>>>>>>>>> partial checksum.
> > > >>>>>>>>>>>>>>>>>> +\item[VIRTIO_NET_F_GUEST_CSUM (1)] Driver handles packets with
> > > >>>>>>>>>>>>>>>>>> partial checksum or full checksum.
> > > >>>>>>>>>>>>>>>>> So patch 2 said
> > > >>>>>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>>>>> "
> > > >>>>>>>>>>>>>>>>> +\item[VIRTIO_NET_F_GUEST_FULL_CSUM (64)] Driver handles packets with
> > > >>>>>>>>>>>>>>>>> full checksum.
> > > >>>>>>>>>>>>>>>>> \end{description}
> > > >>>>>>>>>>>>>>>>> "
> > > >>>>>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>>>>> Is there any difference between the two "full checksum" here?
> > > >>>>>>>>>>>>>>>> There's no difference.
> > > >>>>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>>>> The core is that VIRTIO_NET_F_GUEST_FULL_CSUM means that the driver
> > > >>>>>>>>>>>>>>>> "can
> > > >>>>>>>>>>>>>>>> only" handle packets with full checksum.
> > > >>>>>>>>>>>>>>> This seems to be odd.
> > > >>>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>>> Driver can always handle packet with full checksum, no?
> > > >>>>>>>>>>>>>> Yes.
> > > >>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>>> I meant it
> > > >>>>>>>>>>>>>>> will be then to be functional equivalent to !
> > > >>>>>>>>>>>>>>> VIRTIO_NET_F_GUEST_FULL_CSUM?
> > > >>>>>>>>>>>>>> Are you referring to
> > > >>>>>>>>>>>>>> "functional equivalent to !VIRTIO_NET_F_GUEST_CSUM" ?
> > > >>>>>>>>>>>>> Sorry, this is a typo. I meant
> > > >>>>>>>>>>>>>
> > > >>>>>>>>>>>>> Are you referring to
> > > >>>>>>>>>>>>> "functional equivalent to !VIRTIO_NET_F_GUEST_FULL_CSUM" ?
> > > >>>>>>>>>>>>>
> > > >>>>>>>>>>>>>> If so, I think it's no.
> > > >>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>> Maybe a description similar to the following would be more clearer:
> > > >>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>> +\item[VIRTIO_NET_F_GUEST_FULL_CSUM (64)] Driver does not handle
> > > >>>>>>>>>>>>>> packets with partial checksum.
> > > >>>>>>>>>>>> I may miss something here, but what's the difference between
> > > >>>>>>>>>>>>
> > > >>>>>>>>>>>> VIRTIO_NET_F_GUEST_FULL_CSUM
> > > >>>>>>>>>>>>
> > > >>>>>>>>>>>> and
> > > >>>>>>>>>>>>
> > > >>>>>>>>>>>> !VIRTIO_NET_F_GUEST_CSUM?
> > > >>>>>>>>>>> From the device perspective:
> > > >>>>>>>>>>>
> > > >>>>>>>>>>> If !VIRTIO_NET_F_GUEST_CSUM, the device delivers packets with full
> > > >>>>>>>>>>> checksum to the driver,
> > > >>>>>>>>>>> but the device can not validate the checksum for these packets. That is,
> > > >>>>>>>>>>> the flags in virtio-net-hdr
> > > >>>>>>>>>>> will not contain _DATA_VALID, and the driver or stack needs to validate
> > > >>>>>>>>>>> these packets.
> > > >>>>>>>>>>>
> > > >>>>>>>>>>> If VIRTIO_NET_F_GUEST_FULL_CSUM, the device delivers packets with full
> > > >>>>>>>>>>> checksum to the driver,
> > > >>>>>>>>>>> and the device can validate the checksum for these packets. That is, the
> > > >>>>>>>>>>> flags in virtio-net-hdr
> > > >>>>>>>>>>> will contain _DATA_VALID,
> > > >>>>>>>>>> I think DATA_VALID is optional here as device can't recognize all type
> > > >>>>>>>>>> of protocols.
> > > >>>>>>>>> Yes, you are right, so I used "device *can*" here. Which packet types
> > > >>>>>>>>> the device recognizes or validates
> > > >>>>>>>>> depends on the device's implementation. This is also the current
> > > >>>>>>>>> practice of GUEST_CSUM.
> > > >>>>>>>>>
> > > >>>>>>>>>>> and the driver or stack does not need to
> > > >>>>>>>>>>> validate these packets.
> > > >>>>>>>>>> Ok, so I think there're something that is subtle here,
> > > >>>>>>>>> Ok, I see.
> > > >>>>>>>>>
> > > >>>>>>>>>> and that's why
> > > >>>>>>>>>> I'm asking here:
> > > >>>>>>>>>>
> > > >>>>>>>>>> 1) "Driver does not handle packets with partial checksum" is not
> > > >>>>>>>>>> accurate, !GUEST_CUSM also fit for this definition.
> > > >>>>>>>>>> 2) "Driver handles packets with full checksum" is kind of ambiguous as
> > > >>>>>>>>>> it doesn't say whether or not the packet has been validated or not.
> > > >>>>>>>>> Maybe the description below would be less subtle?
> > > >>>>>>>>> +\item[VIRTIO_NET_F_GUEST_CSUM (1)] Driver handles packets with partial
> > > >>>>>>>>> checksum or full checksum.
> > > >>>>>>>> I'd suggest to leave it as is. As I didn't find any issue since even
> > > >>>>>>>> with DATA_VALID. Did you?
> > > >>>>>>>>
> > > >>>>>>>>> +\item[VIRTIO_NET_F_GUEST_FULL_CSUM (64)] The driver handles packets
> > > >>>>>>>>> with full checksum,
> > > >>>>>>>>> and the device optionally validates the packet's checksum.
> > > >>>>>>>> Or maybe something like (not a native speaker)
> > > >>>>>>>>
> > > >>>>>>>> The driver handles packets with full checksum which the device has
> > > >>>>>>>> already validated.
> > > >>>>>>>>
> > > >>>>>>>> Thanks
> > > >>>>>>> I feel we just need a proper definition of what does "full checksum"
> > > >>>>>>> mean in this context. It is used but not defined.
> > > >>>>>>> Assume this feature was negotiated.
> > > >>>>>>> My understanding is that this is just like VIRTIO_NET_F_GUEST_CSUM
> > > >>>>>>> but certain values in the header are then disallowed? Which?
> > > >>>>>>> This should be in the spec.
> > > >>>>>> Yes, I think it is probably the headers that DATA_VALID can work. We
> > > >>>>>> never define it in the past.
> > > >>>>>>
> > > >>>>>> E.g in the Linux we map DATA_VALID to CHECKSUM_UNNECESSARY, but it can
> > > >>>>>> only work for some specific protocols:
> > > >>>>>>
> > > >>>>>> """
> > > >>>>>> * %CHECKSUM_UNNECESSARY is applicable to following protocols:
> > > >>>>>> *
> > > >>>>>> * - TCP: IPv6 and IPv4.
> > > >>>>>> * - UDP: IPv4 and IPv6. A device may apply CHECKSUM_UNNECESSARY to a
> > > >>>>>> * zero UDP checksum for either IPv4 or IPv6, the networking stack
> > > >>>>>> * may perform further validation in this case.
> > > >>>>>> * - GRE: only if the checksum is present in the header.
> > > >>>>>> * - SCTP: indicates the CRC in SCTP header has been validated.
> > > >>>>>> * - FCOE: indicates the CRC in FC frame has been validated.
> > > >>>>>> """
> > > >>>>>>
> > > >>>>>> I'm not sure whether it's just fine to duplicate the definition or
> > > >>>>>> it's too late to define any now.
> > > >>>>> I think it's mostly harmless for other protocols.
> > > >>>> I'm not sure if this should be defined by a new FULL_CSUM feature.
> > > >>>> This seems to be an issue with GUEST_CSUM.
> > > >>>>
> > > >>>> I think we should supplement these with a new patch for GUEST_CSUM?
> > > >>> Probably. My understanding is:
> > > >>>
> > > >>> You want to reuse DATA_VALID here, so we need to stick to a consistent
> > > >>> semantic for GUEST_CUSM and FULL_CSUM. So we need a definition of
> > > >>> "full csum" or what kind of packet could DATA_VALID work here.
> > > >> I agree, we can be clear about what types of packets DATA_VALID might
> > > >> cover, e.g. TCP/UDP/GRE/SCTP/FoCE.
> > > >>
> > > >> But I think we also need something like \field{supported_validate_types} to
> > > >> indicate which packet types the device supports validating and setting
> > > >> DATA_VALID,
> > > >> otherwise the device driver that negotiates this feature may fail to live
> > > >> migration.
> > > >> Am I right?
> > > >>
> > > >> I'm not sure how GUEST_CSUM works now as it should also suffer from the
> > > >> above
> > > >> mentioned issues with live migration, but no devices are reporting this
> > > >> right now.
> > > >>
> > > >> Maybe, each device only supports checksum verification for TCP/UDP by
> > > >> default? I don't know.
> > > >> But I hope we can focus on this and get consensus, because our hw release
> > > >> date is coming soon.
> > > >>
> > > >> Thanks a lot!
> > > >
> > > > First, DATA_VALID is not a thing that hardware should ever use.
> > > > It's a hack when packets are passed within host.
> > >
> > > Get here. Thanks!
> >
> > So if I understand correctly, we need a new flag here and define the
> > supported protocols.
>
>
> For what, migration?
No.
It's basically a question of whether or not we want to reuse
DATA_VALID. It has limitations that it only works for some specific
protocols.
If we don't, we need a new flag.
Thanks
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