On Fri, Dec 20, 2024 at 8:56 PM Stanislav Fomichev <stfomichev@xxxxxxxxx> wrote: > > On 12/21, Mina Almasry wrote: > > Add documentation outlining the usage and details of the devmem TCP TX > > API. > > > > Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx> > > --- > > 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@xxxxxxxxxx/ > > > > > > -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. > > + > > [...] > > > +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
