Re: [virtio-dev] Re: [virtio-comment] Re: [virtio-dev] Re: [virtio-comment] Re: [PATCH v5] virtio-net: device does not deliver partially checksummed packet and may validate the checksum

[Jason Wang <jasowang@redhat.com>]

On Thu, Dec 21, 2023 at 11:51âAM Heng Qi <hengqi@linux.alibaba.com> wrote:
>
>
>
> å 2023/12/21 äå9:41, Jason Wang åé:
> > On Wed, Dec 20, 2023 at 5:31âPM Heng Qi <hengqi@linux.alibaba.com> wrote:
> >>
> >>
> >> å 2023/12/20 äå3:35, Michael S. Tsirkin åé:
> >>> On Wed, Dec 20, 2023 at 02:30:01PM +0800, Heng Qi wrote:
> >>>> But why are we discussing this?
> >>> I think basically at this point everyone is confused about what
> >>> the feature does. right now we have packets
> >>> with
> >>> #define VIRTIO_NET_HDR_F_NEEDS_CSUM     1       -> partial
> >>> #define VIRTIO_NET_HDR_F_DATA_VALID     2       -> unnecessary
> >>> and packets without either                    -> none
> >>>
> >>> if both 1 and 2 are set then linux uses VIRTIO_NET_HDR_F_NEEDS_CSUM but
> >>> I am not sure it's not a mistake. Maybe it does not matter.
> >>>
> >>> What does this new thing do? So far all we have is "XDP will turn it on"
> >>> which is not really sufficient. I assumed it somehow replaces
> >>> partial with complete. That would make sense for many reasons,
> >>> for example the checksum fields in the header can be reused
> >>> for other purposes. But maybe not?
> >>
> >> Hello Jaosn and Michael. I've summarized our discussion so far, so check
> >> it out below. Thank you very much!
> >>
> >>   From the nic perspective, I think Jason's statement is correct, the
> >> nic's checksum capability and setting DATA_VALID in flags
> >> should not be determined by GUEST_CSUM feature. As long as the rx
> >> checksum offload is turned on, DATA_VALID
> >> should be set. (Though we now bind GUEST_CSUM negotiation with rx
> >> checksum offload.)
> > I think we can fix this in the driver. Probably by just advertising
> > RXCSUM regardless of GUEST_CSUM?
>
> Right.
>
> >
> >> Therefore, we need to pay attention to the information of rx checksum
> >> offload. Please check it out:
> >>
> >> Devices that comply with the below description are said to be existing
> >> devices:
> >>       "If VIRTIO_NET_F_GUEST_CSUM is not negotiated, the device *MUST*
> >> set flags to zero and SHOULD supply a fully checksummed packet to the
> >> driver."
> >>
> >> As suggested by Jason, devices that comply with the below description
> >> are said to be new devices:
> >>       "If VIRTIO_NET_F_GUEST_CSUM is not negotiated, the device *MAY* set
> >> flags to zero and SHOULD supply a fully checksummed packet to the driver."
> >>
> >>
> >> 1. Rx checksum offload is turned on
> >> GUEST_CSUM feature is not negotiated. (now it is only used to indicate
> >> whether the driver can handle partially checksummed packets)
> >>      a. Existing devices continue to set flags to 0;
> > Note that existing devices can set DATA_VALID regardless of rx csum.
>
> Right.
>
> >
> >>      b. New devices may validate the packets and have flags set to
> >> DATA_VALID;
> >>      c. Migration.
> >>          Migration of existing devices continues to check GUEST_CSUM
> >> feature and rx checksum offload;
> >>          Migration of new devices only check rx checksum offload;
> >>          Without updating the existing migration management and control
> >> system, existing devices cannot be migrated to new devices, and new
> >> devices cannot be migrated to existing devices.
> > Yes.
> >
> >>      d. How offload should be controlled now needs attention. Should
> >> CTRL_GUEST_OFFLOADS still issue GUEST_CSUM feature bit to control the rx
> >> checksum offload?
> > So the only thing we need to do for the driver is, when rx csum is disabled:
> >
> > 1) drop packets with NEEDS_CSUM
> > 2) use CHECKSUM_NONE for the rest
> >
> > ?
>
> YES.
>
> >
> >> 2. The new FULLY_CSUM feature must disable NEEDS_CSUM.
> >> The device may set DATA_VALID regardless of whether FULLY_CSUM or
> >> GUEST_CSUM is negotiated.
> >>      a. Rx fully checksum offload is still controlled by
> >> CTRL_GUEST_OFFLOADS carrying GUEST_FULLY_CSUM.
> >>      b. When the rx device receives a partially checksummed packet, it
> >> should calculate the checksum and delivering a fully checksummed packet
> >> to the driver.
> >>
> >>
> >> So now, if we modify the existing spec as Jason suggested, I think it's OK.
> >> But we need to find out how to control rx checksum offload. WDYT?
> > See above, the driver can just not set CHECKSUM_UNNECESSARY in this case.
>
> I think what you are saying here is that CHECKSUM_UNNECESSARY cannot be
> set by the driver when rx checksum offload is turned off.
>
> Thanksï

Right.

Thanks

>
> >
> > Thanks
> >
> >> Thanks!
> >>
> >>>
>