On 2019-06-13 20:29, Jakub Kicinski wrote: > On Thu, 13 Jun 2019 14:01:39 +0000, Maxim Mikityanskiy wrote: >> On 2019-06-12 23:10, Jakub Kicinski wrote: >>> On Wed, 12 Jun 2019 15:56:43 +0000, Maxim Mikityanskiy wrote: >>>> The typical XDP memory scheme is one packet per page. Change the AF_XDP >>>> frame size in libbpf to 4096, which is the page size on x86, to allow >>>> libbpf to be used with the drivers with the packet-per-page scheme. >>> >>> This is slightly surprising. Why does the driver care about the bufsz? >> >> The classic XDP implementation supports only the packet-per-page scheme. >> mlx5e implements this scheme, because it perfectly fits with xdp_return >> and page pool APIs. AF_XDP relies on XDP, and even though AF_XDP doesn't >> really allocate or release pages, it works on top of XDP, and XDP >> implementation in mlx5e does allocate and release pages (in general >> case) and works with the packet-per-page scheme. > > Yes, okay, I get that. But I still don't know what's the exact use you > have for AF_XDP buffers being 4k.. Could you point us in the code to > the place which relies on all buffers being 4k in any XDP scenario? 1. An XDP program is set on all queues, so to support non-4k AF_XDP frames, we would also need to support multiple-packet-per-page XDP for regular queues. 2. Page allocation in mlx5e perfectly fits page-sized XDP frames. Some examples in the code are: 2.1. mlx5e_free_rx_mpwqe calls a generic mlx5e_page_release to release the pages of a MPWQE (multi-packet work queue element), which is implemented as xsk_umem_fq_reuse for the case of XSK. We avoid extra overhead by using the fact that packet == page. 2.2. mlx5e_free_xdpsq_desc performs cleanup after XDP transmits. In case of XDP_TX, we can free/recycle the pages without having a refcount overhead, by using the fact that packet == page. >>> You're not supposed to so page operations on UMEM pages, anyway. >>> And the RX size filter should be configured according to MTU regardless >>> of XDP state. >> >> Yes, of course, MTU is taken into account. >> >>> Can you explain? >>> >>>> Add a command line option -f to xdpsock to allow to specify a custom >>>> frame size. >>>> >>>> Signed-off-by: Maxim Mikityanskiy <maximmi@xxxxxxxxxxxx> >>>> Reviewed-by: Tariq Toukan <tariqt@xxxxxxxxxxxx> >>>> Acked-by: Saeed Mahameed <saeedm@xxxxxxxxxxxx>
