Re: [PATCH bpf-next v2] libbpf: use implicit XSKMAP lookup from AF_XDP XDP program

["Björn Töpel" <bjorn.topel@gmail.com>]

On Mon, 21 Oct 2019 at 13:50, Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Björn Töpel <bjorn.topel@gmail.com> writes:
>
> > From: Björn Töpel <bjorn.topel@intel.com>
> >
> > In commit 43e74c0267a3 ("bpf_xdp_redirect_map: Perform map lookup in
> > eBPF helper") the bpf_redirect_map() helper learned to do map lookup,
> > which means that the explicit lookup in the XDP program for AF_XDP is
> > not needed for post-5.3 kernels.
> >
> > This commit adds the implicit map lookup with default action, which
> > improves the performance for the "rx_drop" [1] scenario with ~4%.
> >
> > For pre-5.3 kernels, the bpf_redirect_map() returns XDP_ABORTED, and a
> > fallback path for backward compatibility is entered, where explicit
> > lookup is still performed. This means a slight regression for older
> > kernels (an additional bpf_redirect_map() call), but I consider that a
> > fair punishment for users not upgrading their kernels. ;-)
> >
> > v1->v2: Backward compatibility (Toke) [2]
> >
> > [1] # xdpsock -i eth0 -z -r
> > [2] https://lore.kernel.org/bpf/87pnirb3dc.fsf@toke.dk/
> >
> > Suggested-by: Toke Høiland-Jørgensen <toke@redhat.com>
> > Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
> > ---
> >  tools/lib/bpf/xsk.c | 45 +++++++++++++++++++++++++++++++++++----------
> >  1 file changed, 35 insertions(+), 10 deletions(-)
> >
> > diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> > index b0f532544c91..391a126b3fd8 100644
> > --- a/tools/lib/bpf/xsk.c
> > +++ b/tools/lib/bpf/xsk.c
> > @@ -274,33 +274,58 @@ static int xsk_load_xdp_prog(struct xsk_socket *xsk)
> >       /* This is the C-program:
> >        * SEC("xdp_sock") int xdp_sock_prog(struct xdp_md *ctx)
> >        * {
> > -      *     int index = ctx->rx_queue_index;
> > +      *     int ret, index = ctx->rx_queue_index;
> >        *
> >        *     // A set entry here means that the correspnding queue_id
> >        *     // has an active AF_XDP socket bound to it.
> > +      *     ret = bpf_redirect_map(&xsks_map, index, XDP_PASS);
> > +      *     ret &= XDP_PASS | XDP_REDIRECT;
>
> Why the masking? Looks a bit weird (XDP return codes are not defined as
> bitmask values), and it's not really needed, is it?
>

bpf_redirect_map() returns a 32-bit signed int, so the upper 32-bit
will need to be cleared. Having an explicit AND is one instruction
less than two shifts. So, it's an optimization (every instruction is
sacred).

Compare these two:

0000000000000000 xdp_sock_prog:
;     int ret, index = ctx->rx_queue_index;
       0:       61 12 10 00 00 00 00 00 r2 = *(u32 *)(r1 + 16)
       1:       63 2a fc ff 00 00 00 00 *(u32 *)(r10 - 4) = r2
;     ret = bpf_redirect_map(&xsks_map, index, XDP_PASS);
       2:       18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
       4:       b7 03 00 00 02 00 00 00 r3 = 2
       5:       85 00 00 00 33 00 00 00 call 51
;     ret &= XDP_PASS | XDP_REDIRECT;
       6:       57 00 00 00 06 00 00 00 r0 &= 6
;     if (ret)
       7:       55 00 0d 00 00 00 00 00 if r0 != 0 goto +13 <LBB0_3>
       8:       bf a2 00 00 00 00 00 00 r2 = r10
;     if (bpf_map_lookup_elem(&xsks_map, &index))
       9:       07 02 00 00 fc ff ff ff r2 += -4
      10:       18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
      12:       85 00 00 00 01 00 00 00 call 1
      13:       bf 01 00 00 00 00 00 00 r1 = r0
      14:       b7 00 00 00 02 00 00 00 r0 = 2
      15:       15 01 05 00 00 00 00 00 if r1 == 0 goto +5 <LBB0_3>
;         return bpf_redirect_map(&xsks_map, index, 0);
      16:       61 a2 fc ff 00 00 00 00 r2 = *(u32 *)(r10 - 4)
      17:       18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
      19:       b7 03 00 00 00 00 00 00 r3 = 0
      20:       85 00 00 00 33 00 00 00 call 51

00000000000000a8 LBB0_3:
; }
      21:       95 00 00 00 00 00 00 00 exit


Disassembly of section xdp_sock:

0000000000000000 xdp_sock_prog:
;     int ret, index = ctx->rx_queue_index;
       0:       61 12 10 00 00 00 00 00 r2 = *(u32 *)(r1 + 16)
       1:       63 2a fc ff 00 00 00 00 *(u32 *)(r10 - 4) = r2
;     ret = bpf_redirect_map(&xsks_map, index, XDP_PASS);
       2:       18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
       4:       b7 03 00 00 02 00 00 00 r3 = 2
       5:       85 00 00 00 33 00 00 00 call 51
       6:       67 00 00 00 20 00 00 00 r0 <<= 32
       7:       c7 00 00 00 20 00 00 00 r0 s>>= 32
;     if (ret > 0)
       8:       65 00 0d 00 00 00 00 00 if r0 s> 0 goto +13 <LBB0_3>
       9:       bf a2 00 00 00 00 00 00 r2 = r10
;     if (bpf_map_lookup_elem(&xsks_map, &index))
      10:       07 02 00 00 fc ff ff ff r2 += -4
      11:       18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
      13:       85 00 00 00 01 00 00 00 call 1
      14:       bf 01 00 00 00 00 00 00 r1 = r0
      15:       b7 00 00 00 02 00 00 00 r0 = 2
      16:       15 01 05 00 00 00 00 00 if r1 == 0 goto +5 <LBB0_3>
;         return bpf_redirect_map(&xsks_map, index, 0);
      17:       61 a2 fc ff 00 00 00 00 r2 = *(u32 *)(r10 - 4)
      18:       18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
      20:       b7 03 00 00 00 00 00 00 r3 = 0
      21:       85 00 00 00 33 00 00 00 call 51

00000000000000b0 LBB0_3:
; }
      22:       95 00 00 00 00 00 00 00 exit


Björn

> -Toke
>