Re: [virtio-dev] Re: [Proposal] Relationship between XDP and rx-csum in virtio-net

Heng Qi
2023-05-23T09:18:30Z

On Tue, May 23, 2023 at 03:15:37AM -0400, Michael S. Tsirkin wrote:
> On Tue, May 23, 2023 at 10:41:18AM +0800, Heng Qi wrote:
> > On Mon, May 22, 2023 at 03:10:05PM -0400, Michael S. Tsirkin wrote:
> > > On Mon, May 22, 2023 at 08:12:00PM +0800, Heng Qi wrote:
> > > > 1) Add a feature bit to the virtio specification to tell the sender that a fully
> > > > csumed packet must be sent.
> > >
> > > Who is the sender in this picture? The driver?
> >
> > The device or the driver.
> >
> > When the device is hw, the sender is more likely to be a device.
> > When the device is sw, the sender can be a device or a driver.
> >
> > But in general, this feature is inclined to constrain the behavior of the device and
> > the driver from the receiving side.
>
> Based on above I am guessing you are talking about driver getting
> packets from device, I wish you used terms from virtio spec.

Yes, I'm going to use the terminology of the virtio spec.

>
> > For example:
> > VIRTIO_NET_F_UNNECESSARY_CSUM : The driver tells the device that you must send me a fully csumed packet.
> >
> > Then the specific implementation can be
> >
> > (1) the sender sends a fully csumed packet;
> > (2) the receiver receives a CHECKSUM_PARTIAL packet, and the device helps calculate the fully csum
> > (because the two parties in the communication are located on the same host, the packet is trusted.).
> >
> > In summary, if VIRTIO_NET_F_UNNECESSARY_CSUM is negotiated, the driver will no longer receive any packets marked CHECKSUM_PARTIAL.
> >
> > Thanks.
>
> This is what clearing VIRTIO_NET_F_GUEST_CSUM does.

Yes, but with VIRTIO_NET_F_GUEST_CSUM cleared, although the device can
receive a fully checksummed packet, we can no longer enjoy
the device's ability to validate the packet checksum. That is, the value
of \field{flags} in the virtio_net_hdr structure is set to 0, which means
that the packet received by the driver will not be marked as
VIRTIO_NET_HDR_F_DATA_VALID.

So, we need a feature bit (let's say VIRTIO_NET_F_UNNECESSARY_CSUM).
If VIRTIO_NET_F_UNNECESSARY_CSUM is negotiated, the device must give the
driver a fully checksummed packet, and the packet is validated by the
device with \field{flags} set to VIRTIO_NET_HDR_F_DATA_VALID.

>
> I feel you are trying to say that clearing VIRTIO_NET_F_GUEST_CSUM
> disables all offloads but you want to keep some of them?
>

No, what I mean is that a feature VIRTIO_NET_F_UNNECESSARY_CSUM is needed
in addition to VIRTIO_NET_F_GUEST_CSUM, if both features are negotiated,
then the driver may always receive packets marked as
VIRTIO_NET_HDR_F_DATA_VALID, which means that we can now load XDP at the
same time.

> Again please use virtio terminology not Linux. to help you out,
> in current linux, VIRTIO_NET_HDR_F_NEEDS_CSUM and VIRTIO_NET_HDR_F_DATA_VALID
> will set CHECKSUM_PARTIAL and CHECKSUM_UNNECESSARY respectively.
>

Sure. Will do as you suggested.

Thanks.

>
> --
> MST
>
>
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