Re: [PATCH RFC net-next v1 1/5] net: add devmem TCP TX documentation

[Stanislav Fomichev <stfomichev@gmail.com>]

On 01/27, Mina Almasry wrote:
> On Fri, Dec 20, 2024 at 8:56 PM Stanislav Fomichev <stfomichev@gmail.com> 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@google.com>
> > > ---
> > >  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.
> > > +
> >
> > [...]
> >
> > > +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.

Great, thanks for the update, looking forward!