[PATCH net-next 08/10] doc/mctp: Add a little detail about kernel internals
From: Matt Johnston <matt@codeconstruct.com.au>
Date: 2021-09-29 07:27:07
Subsystem:
documentation, management component transport protocol (mctp), networking [general], the rest · Maintainers:
Jonathan Corbet, Jeremy Kerr, Matt Johnston, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds
From: Jeremy Kerr <jk@codeconstruct.com.au> Describe common flows and refcounting behaviour. Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au> --- Documentation/networking/mctp.rst | 59 +++++++++++++++++++++++++++++++ 1 file changed, 59 insertions(+)
diff --git a/Documentation/networking/mctp.rst b/Documentation/networking/mctp.rst
index 6100cdc220f6..2c54b029f990 100644
--- a/Documentation/networking/mctp.rst
+++ b/Documentation/networking/mctp.rst@@ -211,3 +211,62 @@ remote address is already known, or the message does not require a reply. Like the send calls, sockets will only receive responses to requests they have sent (TO=1) and may only respond (TO=0) to requests they have received. + +Kernel internals +================ + +There are a few possible packet flows in the MCTP stack: + +1. local TX to remote endpoint, message <= MTU:: + + sendmsg() + -> mctp_local_output() + : route lookup + -> rt->output() (== mctp_route_output) + -> dev_queue_xmit() + +2. local TX to remote endpoint, message > MTU:: + + sendmsg() + -> mctp_local_output() + -> mctp_do_fragment_route() + : creates packet-sized skbs. For each new skb: + -> rt->output() (== mctp_route_output) + -> dev_queue_xmit() + +3. remote TX to local endpoint, single-packet message:: + + mctp_pkttype_receive() + : route lookup + -> rt->output() (== mctp_route_input) + : sk_key lookup + -> sock_queue_rcv_skb() + +4. remote TX to local endpoint, multiple-packet message:: + + mctp_pkttype_receive() + : route lookup + -> rt->output() (== mctp_route_input) + : sk_key lookup + : stores skb in struct sk_key->reasm_head + + mctp_pkttype_receive() + : route lookup + -> rt->output() (== mctp_route_input) + : sk_key lookup + : finds existing reassembly in sk_key->reasm_head + : appends new fragment + -> sock_queue_rcv_skb() + +Key refcounts +------------- + + * keys are refed by: + + - a skb: during route output, stored in ``skb->cb``. + + - netns and sock lists. + + * keys can be associated with a device, in which case they hold a + reference to the dev (set through ``key->dev``, counted through + ``dev->key_count``). Multiple keys can reference the device.
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2.30.2