On Mon, 21 Oct 2019 at 13:50, Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
>
> Björn Töpel <bjorn.topel@xxxxxxxxx> writes:
>
> > From: Björn Töpel <bjorn.topel@xxxxxxxxx>
> >
> > 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@xxxxxxx/
> >
> > Suggested-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
> > Signed-off-by: Björn Töpel <bjorn.topel@xxxxxxxxx>
> > ---
> > 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
>
