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.
+
+The user passes the dma-buf id via the dmabuf_tx_cmsg.dmabuf_id, and passes the
+offset into the dmabuf from where to start sending using the
+dmabuf_tx_cmsg.dmabuf_offset field::
+
+        char ctrl_data[CMSG_SPACE(sizeof(struct dmabuf_tx_cmsg))];
+        struct dmabuf_tx_cmsg ddmabuf;
+        struct msghdr msg = {};
+        struct cmsghdr *cmsg;
+        uint64_t off = 100;
+        struct iovec iov;
+
+	iov.iov_base = NULL;
+	iov.iov_len = line_size;
+
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+
+	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;
+	ddmabuf.dmabuf_offset = off;
+
+	*((struct dmabuf_tx_cmsg *)CMSG_DATA(cmsg)) = ddmabuf;
+
+	ret = sendmsg(socket_fd, &msg, MSG_ZEROCOPY);
+
+Reusing TX dmabufs
+------------------
+
+Similar to MSG_ZEROCOPY with regular memory, the user should not modify the
+contents of the dma-buf while a send operation is in progress. This is because
+the kernel does not keep a copy of the dmabuf contents. Instead, the kernel
+will pin and send data from the buffer available to the userspace.
+
+Just as in MSG_ZEROCOPY, the kernel notifies the userspace of send completions
+using MSG_ERRQUEUE::
+
+        int64_t tstop = gettimeofday_ms() + waittime_ms;
+        char control[CMSG_SPACE(100)] = {};
+        struct sock_extended_err *serr;
+        struct msghdr msg = {};
+        struct cmsghdr *cm;
+        int retries = 10;
+        __u32 hi, lo;
+
+        msg.msg_control = control;
+        msg.msg_controllen = sizeof(control);
+
+        while (gettimeofday_ms() < tstop) {
+                if (!do_poll(fd)) continue;
+
+                ret = recvmsg(fd, &msg, MSG_ERRQUEUE);
+
+                for (cm = CMSG_FIRSTHDR(&msg); cm; cm = CMSG_NXTHDR(&msg, cm)) {
+                        serr = (void *)CMSG_DATA(cm);
+
+                        hi = serr->ee_data;
+                        lo = serr->ee_info;
+
+                        fprintf(stdout, "tx complete [%d,%d]\n", lo, hi);
+                }
+        }
+
+After the associated sendmsg has been completed, the dmabuf can be reused by
+the userspace.
+
+
 Implementation & Caveats
 ========================
 
-- 
2.47.1.613.gc27f4b7a9f-goog

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?

> +The user passes the dma-buf id via the dmabuf_tx_cmsg.dmabuf_id, and passes the
> +offset into the dmabuf from where to start sending using the
> +dmabuf_tx_cmsg.dmabuf_offset field::
> +

[...]

> +        char ctrl_data[CMSG_SPACE(sizeof(struct dmabuf_tx_cmsg))];
> +        struct dmabuf_tx_cmsg ddmabuf;
> +        struct msghdr msg = {};
> +        struct cmsghdr *cmsg;
> +        uint64_t off = 100;
> +        struct iovec iov;
> +
> +	iov.iov_base = NULL;
> +	iov.iov_len = line_size;

nit: indent seems to be different (tabs vs spaces)

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.

-- 
Thanks,
Mina

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!