[xdp-hints] Re: [PATCH bpf-next v5 02/13] xsk: Add TX timestamp and TX checksum offload support

[Stanislav Fomichev <sdf@google.com>]

On Mon, Nov 13, 2023 at 5:16 AM Jesper Dangaard Brouer <hawk@kernel.org> wrote:
>
>
>
> On 11/2/23 23:58, Stanislav Fomichev wrote:
> > diff --git a/include/uapi/linux/if_xdp.h b/include/uapi/linux/if_xdp.h
> > index 2ecf79282c26..b0ee7ad19b51 100644
> > --- a/include/uapi/linux/if_xdp.h
> > +++ b/include/uapi/linux/if_xdp.h
> > @@ -106,6 +106,41 @@ struct xdp_options {
> >   #define XSK_UNALIGNED_BUF_ADDR_MASK \
> >       ((1ULL << XSK_UNALIGNED_BUF_OFFSET_SHIFT) - 1)
> >
> > +/* Request transmit timestamp. Upon completion, put it into tx_timestamp
> > + * field of struct xsk_tx_metadata.
> > + */
> > +#define XDP_TXMD_FLAGS_TIMESTAMP             (1 << 0)
> > +
> > +/* Request transmit checksum offload. Checksum start position and offset
> > + * are communicated via csum_start and csum_offset fields of struct
> > + * xsk_tx_metadata.
> > + */
> > +#define XDP_TXMD_FLAGS_CHECKSUM                      (1 << 1)
> > +
> > +/* AF_XDP offloads request. 'request' union member is consumed by the driver
> > + * when the packet is being transmitted. 'completion' union member is
> > + * filled by the driver when the transmit completion arrives.
> > + */
> > +struct xsk_tx_metadata {
> > +     union {
> > +             struct {
> > +                     __u32 flags;
> > +
> > +                     /* XDP_TXMD_FLAGS_CHECKSUM */
> > +
> > +                     /* Offset from desc->addr where checksumming should start. */
> > +                     __u16 csum_start;
> > +                     /* Offset from csum_start where checksum should be stored. */
> > +                     __u16 csum_offset;
> > +             } request;
> > +
> > +             struct {
> > +                     /* XDP_TXMD_FLAGS_TIMESTAMP */
> > +                     __u64 tx_timestamp;
> > +             } completion;
> > +     };
> > +};
>
> This looks wrong to me. It looks like member @flags is not avail at
> completion time.  At completion time, I assume we also want to know if
> someone requested to get the timestamp for this packet (else we could
> read garbage).

I've moved the parts that are preserved across tx and tx completion
into xsk_tx_metadata_compl.
This is to address Magnus/Maciej feedback where userspace might race
with the kernel.
See: https://lore.kernel.org/bpf/ZNoJenzKXW5QSR3E@boxer/

> Another thing (I've raised this before): It would be really practical to
> store an u64 opaque value at TX and then read it at Completion time.
> One use-case is a forwarding application storing HW RX-time and
> comparing this to TX completion time to deduce the time spend processing
> the packet.

This can be another member, right? But note that extending
xsk_tx_metadata_compl might be a bit complicated because drivers have
to carry this info somewhere. So we have to balance the amount of
passed data between the tx and the completion.