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

[Heng Qi <hengqi@linux.alibaba.com>]

On Tue, May 23, 2023 at 09:30:28AM -0400, Michael S. Tsirkin wrote:
> On Tue, May 23, 2023 at 05:18:20PM +0800, Heng Qi wrote:
> > 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.
> 
> Makes no sense to me. VIRTIO_NET_F_GUEST_CSUM set already allows
> VIRTIO_NET_HDR_F_DATA_VALID:

We need to focus on what happens when the XDP program is loaded:

The current virtio-net needs to turn off the VIRTIO_NET_F_GUEST_CSUM
feature when loading XDP. The main reason for doing this is because
VIRTIO_NET_F_GUEST_CSUM allows to receive packets marked as
VIRTIO_NET_HDR_F_NEEDS_CSUM. Such packets are not compatible with
XDP programs, because we cannot guarantee that the csum_{start, offset}
fields are correct after XDP modifies the packets.

So in order for the driver to continue to enjoy the device's ability to
validate packets while XDP is loaded, we need a new feature to tell the
device not to deliver packets marked as VIRTIO_NET_HDR_F_NEEDS_CSUM (in
other words, the device can only deliver packets marked as
VIRTIO_NET_HDR_F_DATA_VALID).

Thanks.

> \item If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the
>   VIRTIO_NET_HDR_F_NEEDS_CSUM bit in \field{flags} can be
>   set: if so, the packet checksum at offset \field{csum_offset}
>   from \field{csum_start} and any preceding checksums
>   have been validated.  The checksum on the packet is incomplete and
>   if bit VIRTIO_NET_HDR_F_RSC_INFO is not set in \field{flags}, 
>   then \field{csum_start} and \field{csum_offset} indicate how to calculate it
>   (see Packet Transmission point 1).
> 
> Did you maybe mean if either feature is negotiated?
> 
> 
> 
> > > 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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