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

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Date:   Fri, 19 May 2017 21:13:14 -0700
From:   Jakub Kicinski <kubakici@...pl>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Daniel Borkmann <borkmann@...earbox.net>,
        netdev@...r.kernel.org
Subject: Re: [RFC net-next PATCH 4/5] net: new XDP feature for reading HW
 rxhash from drivers

On Fri, 19 May 2017 20:34:00 -0700, Alexei Starovoitov wrote:
> 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.

Ack, quick read of tracing code reveals this indeed should work.