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xsk: Extend documentation for Rx|Tx-only sockets and shared umems 

Add more documentation about the new Rx-only and Tx-only sockets in
libbpf and also how libbpf can now support shared umems. Also found
two pieces that could be improved in the text, that got fixed in this
commit.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: William Tu <u9012063@gmail.com>
Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Link: https://lore.kernel.org/bpf/1573148860-30254-6-git-send-email-magnus.karlsson@intel.com
alistair/sunxi64-5.5-dsi
Magnus Karlsson 2019-11-07 18:47:40 +01:00 committed by Alexei Starovoitov
parent 661842c46d
commit 57afa8b0cf
1 changed files with 23 additions and 5 deletions
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28
Documentation/networking/af_xdp.rst
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@ -295,7 +295,7 @@ round-robin example of distributing packets is shown below:
{
rr = (rr + 1) & (MAX_SOCKS - 1);
return bpf_redirect_map(&xsks_map, rr, 0);
return bpf_redirect_map(&xsks_map, rr, XDP_DROP);
}
Note, that since there is only a single set of FILL and COMPLETION
@ -304,6 +304,12 @@ to make sure that multiple processes or threads do not use these rings
concurrently. There are no synchronization primitives in the
libbpf code that protects multiple users at this point in time.
Libbpf uses this mode if you create more than one socket tied to the
same umem. However, note that you need to supply the
XSK_LIBBPF_FLAGS__INHIBIT_PROG_LOAD libbpf_flag with the
xsk_socket__create calls and load your own XDP program as there is no
built in one in libbpf that will route the traffic for you.
XDP_USE_NEED_WAKEUP bind flag
-----------------------------
@ -355,10 +361,22 @@ to set the size of at least one of the RX and TX rings. If you set
both, you will be able to both receive and send traffic from your
application, but if you only want to do one of them, you can save
resources by only setting up one of them. Both the FILL ring and the
COMPLETION ring are mandatory if you have a UMEM tied to your socket,
which is the normal case. But if the XDP_SHARED_UMEM flag is used, any
socket after the first one does not have a UMEM and should in that
case not have any FILL or COMPLETION rings created.
COMPLETION ring are mandatory as you need to have a UMEM tied to your
socket. But if the XDP_SHARED_UMEM flag is used, any socket after the
first one does not have a UMEM and should in that case not have any
FILL or COMPLETION rings created as the ones from the shared umem will
be used. Note, that the rings are single-producer single-consumer, so
do not try to access them from multiple processes at the same
time. See the XDP_SHARED_UMEM section.
In libbpf, you can create Rx-only and Tx-only sockets by supplying
NULL to the rx and tx arguments, respectively, to the
xsk_socket__create function.
If you create a Tx-only socket, we recommend that you do not put any
packets on the fill ring. If you do this, drivers might think you are
going to receive something when you in fact will not, and this can
negatively impact performance.
XDP_UMEM_REG setsockopt
-----------------------