Re: [RFC PATCH v2 13/21] net/tcp: Pad TCP options out to a fixed size for netgpu

[Jonathan Lemon <jonathan.lemon@gmail.com>]

On Mon, Jul 27, 2020 at 08:19:24AM -0700, Eric Dumazet wrote:
> On Mon, Jul 27, 2020 at 12:51 AM Jonathan Lemon
> <jonathan.lemon@gmail.com> wrote:
> >
> > From: Jonathan Lemon <bsd@fb.com>
> >
> > The "header splitting" feature used by netgpu doesn't actually parse
> > the incoming packet header.  Instead, it splits the packet at a fixed
> > offset.  In order for this to work, the sender needs to send packets
> > with a fixed header size.
> >
> > Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
> > ---
> >  net/ipv4/tcp_output.c | 20 ++++++++++++++++++++
> >  1 file changed, 20 insertions(+)
> >
> > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> > index d8f16f6a9b02..e8a74d0f7ad2 100644
> > --- a/net/ipv4/tcp_output.c
> > +++ b/net/ipv4/tcp_output.c
> > @@ -438,6 +438,7 @@ struct tcp_out_options {
> >         u8 ws;                  /* window scale, 0 to disable */
> >         u8 num_sack_blocks;     /* number of SACK blocks to include */
> >         u8 hash_size;           /* bytes in hash_location */
> > +       u8 pad_size;            /* additional nops for padding */
> >         __u8 *hash_location;    /* temporary pointer, overloaded */
> >         __u32 tsval, tsecr;     /* need to include OPTION_TS */
> >         struct tcp_fastopen_cookie *fastopen_cookie;    /* Fast open cookie */
> > @@ -562,6 +563,17 @@ static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
> >         smc_options_write(ptr, &options);
> >
> >         mptcp_options_write(ptr, opts);
> > +
> > +#if IS_ENABLED(CONFIG_NETGPU)
> > +       /* pad out options */
> > +       if (opts->pad_size) {
> > +               int len = opts->pad_size;
> > +               u8 *p = (u8 *)ptr;
> > +
> > +               while (len--)
> > +                       *p++ = TCPOPT_NOP;
> > +       }
> > +#endif
> >  }
> >
> >  static void smc_set_option(const struct tcp_sock *tp,
> > @@ -826,6 +838,14 @@ static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb
> >                         opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
> >         }
> >
> > +#if IS_ENABLED(CONFIG_NETGPU)
> > +       /* force padding */
> > +       if (size < 20) {
> > +               opts->pad_size = 20 - size;
> > +               size += opts->pad_size;
> > +       }
> > +#endif
> > +
> 
> This is obviously wrong, as any kernel compiled with CONFIG_NETGPU
> will fail all packetdrill tests suite.
> 
> Also the fixed 20 value is not pretty.

Would changing this into a sysctl be a suitable solution?  It really is
a temporary solution to handle hardware that doesn't support splitting,
and adding a sysctl seems so permanent.....  
-- 
Jonathan