On Fri, Dec 20, 2024 at 8:56 PM Stanislav Fomichev [off-list ref] wrote:

On 12/21, Mina Almasry wrote:

quoted

Add documentation outlining the usage and details of the devmem TCP TX
API.

Signed-off-by: Mina Almasry <redacted>
---
 Documentation/networking/devmem.rst | 140 +++++++++++++++++++++++++++-
 1 file changed, 136 insertions(+), 4 deletions(-)

diff --git a/Documentation/networking/devmem.rst b/Documentation/networking/devmem.rst
index d95363645331..9be01cd96ee2 100644
--- a/Documentation/networking/devmem.rst
+++ b/Documentation/networking/devmem.rst

@@ -62,15 +62,15 @@ More Info
     https://lore.kernel.org/netdev/20240831004313.3713467-1-almasrymina@google.com/


-Interface
-=========
+RX Interface
+============


 Example
 -------

-tools/testing/selftests/net/ncdevmem.c:do_server shows an example of setting up
-the RX path of this API.
+./tools/testing/selftests/drivers/net/hw/ncdevmem:do_server shows an example of
+setting up the RX path of this API.


 NIC Setup

@@ -235,6 +235,138 @@ can be less than the tokens provided by the user in case of:
 (a) an internal kernel leak bug.
 (b) the user passed more than 1024 frags.

+TX Interface
+============
+
+
+Example
+-------
+
+./tools/testing/selftests/drivers/net/hw/ncdevmem:do_client shows an example of
+setting up the TX path of this API.
+
+
+NIC Setup
+---------
+
+The user must bind a TX dmabuf to a given NIC using the netlink API::
+
+        struct netdev_bind_tx_req *req = NULL;
+        struct netdev_bind_tx_rsp *rsp = NULL;
+        struct ynl_error yerr;
+
+        *ys = ynl_sock_create(&ynl_netdev_family, &yerr);
+
+        req = netdev_bind_tx_req_alloc();
+        netdev_bind_tx_req_set_ifindex(req, ifindex);
+        netdev_bind_tx_req_set_fd(req, dmabuf_fd);
+
+        rsp = netdev_bind_tx(*ys, req);
+
+        tx_dmabuf_id = rsp->id;
+
+
+The netlink API returns a dmabuf_id: a unique ID that refers to this dmabuf
+that has been bound.
+
+The user can unbind the dmabuf from the netdevice by closing the netlink socket
+that established the binding. We do this so that the binding is automatically
+unbound even if the userspace process crashes.
+
+Note that any reasonably well-behaved dmabuf from any exporter should work with
+devmem TCP, even if the dmabuf is not actually backed by devmem. An example of
+this is udmabuf, which wraps user memory (non-devmem) in a dmabuf.
+
+Socket Setup
+------------
+
+The user application must use MSG_ZEROCOPY flag when sending devmem TCP. Devmem
+cannot be copied by the kernel, so the semantics of the devmem TX are similar
+to the semantics of MSG_ZEROCOPY.
+
+     ret = setsockopt(socket_fd, SOL_SOCKET, SO_ZEROCOPY, &opt, sizeof(opt));
+
+Sending data
+--------------
+
+Devmem data is sent using the SCM_DEVMEM_DMABUF cmsg.
+

[...]

quoted

+The user should create a msghdr with iov_base set to NULL and iov_len set to the
+number of bytes to be sent from the dmabuf.

Should we verify that iov_base is NULL in the kernel?

But also, alternatively, why not go with iov_base == offset? This way we
can support several offsets in a single message, just like regular
sendmsg with host memory. Any reason to not do that?

Sorry for the late reply. Some of these suggestions took a bit to
investigate and other priorities pulled me a bit from this.

I've prototyped using iov_base as offset with some help from your
published branch, and it works fine. It seems to me a big improvement
to the UAPI. Will reupload RFC v2 while the tree is closed with this
change.

-- 
Thanks,
Mina