Re: [RFC net-next PATCH 4/5] net: new XDP feature for reading HW rxhash from drivers

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Fri, May 19, 2017 at 08:21:47PM -0700, Jakub Kicinski wrote:
> On Fri, 19 May 2017 20:07:52 -0700, Alexei Starovoitov wrote:
> > How about exposing 'struct mlx5_cqe64 *' to XDP programs as-is?
> > We can make sure that XDP program does read only access into it and
> > it will see cqe->rss_hash_result, cqe->rss_hash_type and everything else
> > in there, but this will not be uapi and it will be pretty obvious
> > to program authors that their programs are vendor specific.
> > 'not uapi' here means that mellanox is free to change their HW descriptor
> > and its contents as they wish.
> 
> Hm..  Would that mean we have to teach the verifier about all possible
> drivers and their metadata structures (i.e. sizes thereof).  And add an
> UAPI enum of known drivers?

why? no uapi other than a pointer to this hw rx descriptor.
Different sizeof(hw_rx_descriptor) is not a problem.
We deal with it already in tracing. All tracepoints have different
sizeof(*ctx), yet the safety is preserved.

> Other idea I floated in early days was to standardize the fields but
> let the driver "JIT" the accesses to look at the right offset of the
> right structure.  Admittedly that would be a lot more work.

'standardize the fields' sounds nice, but failed here already.
As far as I can see the meaning of packet 'hash' is quite different
across the drivers and 'hash' is just a beginning.
I hope we can standardize on 'csum' field and make it checksum_complete,
but so far out of 10+G nics only mlx5 and nfp do it in hw.
We need it at least for mlx4, but it can only fake it via expensive math.