Re: [PATCH bpf-next v4 01/15] xsk: fix xsk_umem_xdp_frame_sz()
From: Björn Töpel <hidden>
Date: 2020-05-21 18:06:34
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bpf
On Thu, 21 May 2020 at 06:30, Jesper Dangaard Brouer [off-list ref] wrote:
On Wed, 20 May 2020 16:34:05 +0200 Björn Töpel [off-list ref] wrote:quoted
On 2020-05-20 15:18, Jesper Dangaard Brouer wrote:quoted
On Wed, 20 May 2020 11:47:28 +0200 Björn Töpel [off-list ref] wrote:quoted
From: Björn Töpel <redacted> 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 <redacted> --- 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