On Fri, Nov 4, 2022 at 11:21 AM Stanislav Fomichev <sdf@xxxxxxxxxx> wrote:
>
> On Fri, Nov 4, 2022 at 1:22 AM Björn Töpel <bjorn@xxxxxxxxxx> wrote:
> >
> > Stanislav Fomichev <sdf@xxxxxxxxxx> writes:
> >
> > > While working on a simplified test for [0] it occurred to me that
> > > the following looks fishy:
> > >
> > > data = xsk_umem__get_data(xsk->umem_area, rx_desc->addr);
> > > data_meta = data - sizeof(my metadata);
> > >
> > > Since the data points to umem frame at addr X, data_mem points to
> > > the end of umem frame X-1.
> > >
> > > I don't think it's by design?
> >
> > It is by design. :-)
>
> Noted, thanks for clarifying!
>
> > > diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> > > index 9f0561b67c12..0547fe37ba7e 100644
> > > --- a/net/xdp/xsk.c
> > > +++ b/net/xdp/xsk.c
> > > @@ -163,7 +163,7 @@ static void xsk_copy_xdp(struct xdp_buff *to, struct xdp_buff *from, u32 len)
> > > } else {
> > > from_buf = from->data_meta;
> > > metalen = from->data - from->data_meta;
> > > - to_buf = to->data - metalen;
> >
> > This is to include the XDP meta data in the receive buffer. Note that
> > AF_XDP descriptor that you get back on the RX ring points to the *data*
> > not the metadata.
> >
> > For the unaligned mode you can pass any address (umem offset) into the
> > fill ring, and the kernel will simply mask it and setup headroom
> > accordingly.
>
> Thanks for the details! And what happens in the aligned case?
>
> Looking purely from the user side:
>
> tx_desc = xsk_ring_prod__tx_desc(&xsk->tx, idx);
> tx_desc->addr = idx * UMEM_FRAME_SIZE; /* this has to be aligned to
> the frame size? */
> data = xsk_umem__get_data(xsk->umem_area, tx_desc->addr);
>
> data here is basically = umem_area + idx * UMEM_FRAM_SIZE, right? How
> do I make sure metadata is placed in the same umem chunk? Will passing
> umem headroom do the trick?
Ignore me. I do see now that there is always XDP_PACKET_HEADROOM bytes
of headroom in rx_desc.
>
> > The buffer allocator guarantees that there's XDP_PACKET_HEADROOM
> > available.
> >
> > IOW your example userland code above is correct.
> >
> >
> > Björn
> >
> >
