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 9:41âAM Jason Wang <jasowang@redhat.com> wrote:
>
> 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?
>
> >
> > 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.
>
> >     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
>
> ?
>
> >
> > 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.

For sure, when GUEST_CSUM is enabled, we need to disable it as well.

Thanks

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