Subject: Re: [PATCH RFC net-next v2 1/6] net: add devmem TCP TX documentation

From: Stanislav Fomichev <[email-masked]>
Date: Fri, 31 Jan 2025 09:39:47 -0800

On 01/30, Mina Almasry wrote:
> On Thu, Jan 30, 2025 at 2:59 PM Stanislav Fomichev <[email-masked]> wrote:
> >
> > On 01/30, Mina Almasry wrote:
> > > Add documentation outlining the usage and details of the devmem TCP TX
> > > API.
> > >
> > > Signed-off-by: Mina Almasry <[email-masked]>
> > >
> > > ---
> > >
> > > v2:
> > > - Update documentation for iov_base is the dmabuf offset (Stan)
> > > ---
> > >  Documentation/networking/devmem.rst | 144 +++++++++++++++++++++++++++-
> > >  1 file changed, 140 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/Documentation/networking/devmem.rst b/Documentation/networking/devmem.rst
> > > index d95363645331..8166fe09da13 100644
> > > --- a/Documentation/networking/devmem.rst
> > > +++ b/Documentation/networking/devmem.rst
> > > @@ -62,15 +62,15 @@ More Info
> > >      https://lore.kernel.org/netdev/[email-masked]/
> > >
> > >
> > > -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 +235,142 @@ 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 where,
> > > +
> > > +iov_base is set to the offset into the dmabuf to start sending from.
> > > +iov_len is set to the number of bytes to be sent from the dmabuf.
> > > +
> > > +The user passes the dma-buf id to send from via the dmabuf_tx_cmsg.dmabuf_id.
> > > +
> > > +The example below sends 1024 bytes from offset 100 into the dmabuf, and 2048
> > > +from offset 2000 into the dmabuf. The dmabuf to send from is tx_dmabuf_id::
> > > +
> > > +       char ctrl_data[CMSG_SPACE(sizeof(struct dmabuf_tx_cmsg))];
> > > +       struct dmabuf_tx_cmsg ddmabuf;
> > > +       struct msghdr msg = {};
> > > +       struct cmsghdr *cmsg;
> > > +       struct iovec iov[2];
> > > +
> > > +       iov[0].iov_base = (void*)100;
> > > +       iov[0].iov_len = 1024;
> > > +       iov[1].iov_base = (void*)2000;
> > > +       iov[1].iov_len = 2048;
> > > +
> > > +       msg.msg_iov = iov;
> > > +       msg.msg_iovlen = 2;
> > > +
> > > +       msg.msg_control = ctrl_data;
> > > +       msg.msg_controllen = sizeof(ctrl_data);
> > > +
> > > +       cmsg = CMSG_FIRSTHDR(&msg);
> > > +       cmsg->cmsg_level = SOL_SOCKET;
> > > +       cmsg->cmsg_type = SCM_DEVMEM_DMABUF;
> > > +       cmsg->cmsg_len = CMSG_LEN(sizeof(struct dmabuf_tx_cmsg));
> > > +
> > > +       ddmabuf.dmabuf_id = tx_dmabuf_id;
> > > +
> > > +       *((struct dmabuf_tx_cmsg *)CMSG_DATA(cmsg)) = ddmabuf;
> >
> > [..]
> >
> > > +       sendmsg(socket_fd, &msg, MSG_ZEROCOPY);
> >
> > Not super important, but any reason not to use MSG_SOCK_DEVMEM as a
> > flag? We already use it for recvmsg, seems logical to mirror the same
> > flag on the transmit side?
> 
> Only to remove redundancy, and the possible confusion that could
> arise, and the extra checks needed to catch invalid input.
> 
> With this, the user tells the kernel to send from the dmabuf by
> supplying the SCM_DEVMEM_DMABUF cmsg. If we add another signal like
> MSG_SOCK_DEVMEM, there is room for the user to supply the cmg but not
> the flag (confusion), and the kernel needs to have the code and
> overhead to check that both the flag and the cmsg are provided.

SG! Having another 'if' doesn't seem like a big overhead to me (given that
we already spend a ton of time copying all the cmsg stuff), but no
strong preference on my side..