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Subject: Re: [virtio-comment] Re: [PATCH v4 1/2] virtio-net: update description for VIRTIO_NET_F_GUEST_CSUM.
On Tue, Dec 5, 2023 at 10:18âPM Heng Qi <hengqi@linux.alibaba.com> 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?
Probably, but I think most devices only do this for TCP/UDP packets
now, so we are fine.
>
> 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?
Maybe.
Thanks
> 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!
>
> >
> > Thanks
> >
> >> Thanks!
> >>
> >>>>> After this is written up we will come up with a good short
> >>>>> description for the feature bit.
> >>>>>
> >>>> Thanks
> >>>>
> >>>>>>> Thanks!
> >>>>>>>
> >>>>>>>> Thanks
> >>>>>>>>
> >>>>>>>>> Thanks!
> >>>>>>>>>
> >>>>>>>>>> Thanks
> >>>>>>>>>>
> >>>>>>>>>>>> Thanks!
> >>>>>>>>>>>>
> >>>>>>>>>>>>> Thanks
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>> Thanks!
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> Thanks
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>> \item[VIRTIO_NET_F_CTRL_GUEST_OFFLOADS (2)] Control channel offloads
> >>>>>>>>>>>>>>>> reconfiguration support.
> >>>>>>>>>>>>>>>> --
> >>>>>>>>>>>>>>>> 2.19.1.6.gb485710b
> >>>>>>>>>>>>>>>>
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