On Thu, 21 May 2020 at 06:30, Jesper Dangaard Brouer <brouer@xxxxxxxxxx> wrote:
>
> On Wed, 20 May 2020 16:34:05 +0200
> Björn Töpel <bjorn.topel@xxxxxxxxx> wrote:
>
> > On 2020-05-20 15:18, Jesper Dangaard Brouer wrote:
> > > On Wed, 20 May 2020 11:47:28 +0200
> > > Björn Töpel <bjorn.topel@xxxxxxxxx> wrote:
> > >
> > >> From: Björn Töpel <bjorn.topel@xxxxxxxxx>
> > >>
> > >> Calculating the "data_hard_end" for an XDP buffer coming from AF_XDP
> > >> zero-copy mode, the return value of xsk_umem_xdp_frame_sz() is added
> > >> to "data_hard_start".
> > >>
> > >> Currently, the chunk size of the UMEM is returned by
> > >> xsk_umem_xdp_frame_sz(). This is not correct, if the fixed UMEM
> > >> headroom is non-zero. Fix this by returning the chunk_size without the
> > >> UMEM headroom.
> > >>
> > >> Fixes: 2a637c5b1aaf ("xdp: For Intel AF_XDP drivers add XDP frame_sz")
> > >> Signed-off-by: Björn Töpel <bjorn.topel@xxxxxxxxx>
> > >> ---
> > >> include/net/xdp_sock.h | 2 +-
> > >> 1 file changed, 1 insertion(+), 1 deletion(-)
> > >>
> > >> diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
> > >> index abd72de25fa4..6b1137ce1692 100644
> > >> --- a/include/net/xdp_sock.h
> > >> +++ b/include/net/xdp_sock.h
> > >> @@ -239,7 +239,7 @@ static inline u64 xsk_umem_adjust_offset(struct xdp_umem *umem, u64 address,
> > >>
> > >> static inline u32 xsk_umem_xdp_frame_sz(struct xdp_umem *umem)
> > >> {
> > >> - return umem->chunk_size_nohr + umem->headroom;
> > >> + return umem->chunk_size_nohr;
> > >
> > > Hmm, is this correct?
> > >
> > > As you write "xdp_data_hard_end" is calculated as an offset from
> > > xdp->data_hard_start pointer based on the frame_sz. Will your
> > > xdp->data_hard_start + frame_sz point to packet end?
> > >
> >
> > Yes, I believe this is correct.
> >
> > Say that a user uses a chunk size of 2k, and a umem headroom of, say,
> > 64. This means that the kernel should (at least) leave 64B which the
> > kernel shouldn't touch.
> >
> > umem->headroom | XDP_PACKET_HEADROOM | packet | |
> > ^ ^ ^ ^ ^
> > a b c d e
> >
> > a: data_hard_start
> > b: data
> > c: data_end
> > d: data_hard_end, (e - 320)
> > e: hardlimit of chunk, a + umem->chunk_size_nohr
> >
> > Prior this fix the umem->headroom was *included* in frame_sz.
>
> Thanks for the nice ascii art description. I can now see that you are
> right. We should add this kind of documentation, perhaps as a comment
> in the code?
>
Definitely! I'd say both in code, and af_xdp.rst! I'll make a patch.
Thanks for the suggestion!
Cheers,
Björn
