[PATCH v2] net/9p: add vsock transport

[PATCH v2] net/9p: add vsock transport

[skvarlamatus]

From: Matus Skvarla <skvarlamatus@gmail.com>

Add vsock as a transport option for 9P client connections. This
allows mounting 9P filesystems over VM sockets without requiring TCP/IP
networking or additional userspace tools.

The implementation extends trans_fd.c with vsock support, reusing the
existing socket infrastructure. A new p9_fd_create_vsock() function
handles vsock connection setup by parsing the CID from the mount source,
creating an AF_VSOCK socket, and connecting to the specified endpoint.
All other transport operations (close, request, cancel) use the shared
fd transport implementation.

The privport mount option is not currently supported for vsock and
returns -EOPNOTSUPP if specified.

Add CONFIG_NET_9P_VSOCK option that conditionally compiles vsock support
into 9pnet_fd.ko. This follows the pattern where socket-based transports
(TCP, Unix, vsock) share trans_fd.c, while specialized hardware transports
(virtio, xen, rdma) have dedicated files.

Usage:
  mount -t 9p -o trans=vsock[,port=<port>] <CID> /mnt/point

Signed-off-by: Matus Skvarla <skvarlamatus@gmail.com>
---
Changes in v2:
  - Add depends on NET_9P_FD to Kconfig
  - Use #include <uapi/linux/vm_sockets.h>
  - Reject privport option with -EOPNOTSUPP
  - Use fc->net_ns instead of current->nsproxy->net_ns
  - Widen vsock port to u32 to match sockaddr_vm.svm_port
  - Widen p9_fd_opts.port to u32 and port_str buffer accordingly (in p9_fd_create_tcp())
  - Add vsock to Documentation/filesystems/9p.rst
Link to v1: https://lore.kernel.org/v9fs/20260527073447.86538-1-skvarlamatus@gmail.com/
---
 Documentation/filesystems/9p.rst | 12 +++-
 include/net/9p/client.h          | 10 +++-
 net/9p/Kconfig                   | 10 ++++
 net/9p/trans_fd.c                | 97 +++++++++++++++++++++++++++++++-
 4 files changed, 124 insertions(+), 5 deletions(-)

diff --git a/Documentation/filesystems/9p.rst b/Documentation/filesystems/9p.rst
index 3f65db648db0..d1bac30b3c9d 100644
--- a/Documentation/filesystems/9p.rst
+++ b/Documentation/filesystems/9p.rst
@@ -50,6 +50,14 @@ mount points. Each 9P export is seen by the client as a virtio device with an
 associated "mount_tag" property. Available mount tags can be
 seen by reading /sys/bus/virtio/drivers/9pnet_virtio/virtio<n>/mount_tag files.
 
+For server accessible over vsock (VM sockets)::
+
+	mount -t 9p -o trans=vsock <CID> /mnt/9
+
+where CID is the vsock Context ID of the remote endpoint (2 for host from
+a guest VM, 3+ for guest VMs from the host). The port to connect to can be
+specified with the port option (default: 564).
+
 USBG Usage
 ==========
 
@@ -115,8 +123,10 @@ Options
 			========  ============================================
 			unix 	  specifying a named pipe mount point
 			tcp	  specifying a normal TCP/IP connection
-			fd   	  used passed file descriptors for connection
+			fd   	  uses passed file descriptors for connection
                                   (see rfdno and wfdno)
+			vsock	  connect over VM sockets (vsock) using
+				  CID as the mount source
 			virtio	  connect to the next virtio channel available
 				  (from QEMU with trans_virtio module)
 			rdma	  connect to a specified RDMA channel
diff --git a/include/net/9p/client.h b/include/net/9p/client.h
index 55c6cb54bd25..c0eca56c1f5f 100644
--- a/include/net/9p/client.h
+++ b/include/net/9p/client.h
@@ -122,8 +122,12 @@ struct p9_client {
 		struct {
 			u16 port;
 			bool privport;
-
 		} tcp;
+#if IS_ENABLED(CONFIG_NET_9P_VSOCK)
+		struct {
+			u32 port;
+		} vsock;
+#endif
 	} trans_opts;
 
 	struct idr fids;
@@ -151,13 +155,13 @@ struct p9_client_opts {
  * struct p9_fd_opts - per-transport options for fd transport
  * @rfd: file descriptor for reading (trans=fd)
  * @wfd: file descriptor for writing (trans=fd)
- * @port: port to connect to (trans=tcp)
+ * @port: port to connect to (trans=tcp, trans=vsock)
  * @privport: port is privileged
  */
 struct p9_fd_opts {
 	int rfd;
 	int wfd;
-	u16 port;
+	u32 port;
 	bool privport;
 };
 
diff --git a/net/9p/Kconfig b/net/9p/Kconfig
index 22f8c167845d..b0a6342e3475 100644
--- a/net/9p/Kconfig
+++ b/net/9p/Kconfig
@@ -32,6 +32,16 @@ config NET_9P_VIRTIO
 	  This builds support for a transports between
 	  guest partitions and a host partition.
 
+config NET_9P_VSOCK
+	depends on NET_9P_FD
+	depends on VSOCKETS
+	bool "9P VSOCK Transport"
+	help
+	  This builds support for a transport for 9pfs over
+	  virtual sockets (vsock). This is useful for communication
+	  between virtual machines and their host without requiring
+	  TCP/IP networking configuration or additional userspace software.
+
 config NET_9P_XEN
 	depends on XEN
 	select XEN_XENBUS_FRONTEND
diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
index eb685b52aeb2..3e094ece863e 100644
--- a/net/9p/trans_fd.c
+++ b/net/9p/trans_fd.c
@@ -28,6 +28,9 @@
 #include <net/9p/9p.h>
 #include <net/9p/client.h>
 #include <net/9p/transport.h>
+#if IS_ENABLED(CONFIG_NET_9P_VSOCK)
+#include <uapi/linux/vm_sockets.h>
+#endif
 
 #include <linux/syscalls.h> /* killme */
 
@@ -36,6 +39,9 @@
 
 static struct p9_trans_module p9_tcp_trans;
 static struct p9_trans_module p9_fd_trans;
+#if IS_ENABLED(CONFIG_NET_9P_VSOCK)
+static struct p9_trans_module p9_vsock_trans;
+#endif
 
 enum {
 	Rworksched = 1,		/* read work scheduled or running */
@@ -714,6 +720,12 @@ static int p9_fd_show_options(struct seq_file *m, struct p9_client *clnt)
 		if (clnt->trans_opts.fd.wfd != ~0)
 			seq_printf(m, ",wfd=%u", clnt->trans_opts.fd.wfd);
 	}
+#if IS_ENABLED(CONFIG_NET_9P_VSOCK)
+	else if (clnt->trans_mod == &p9_vsock_trans) {
+		if (clnt->trans_opts.vsock.port != P9_FD_PORT)
+			seq_printf(m, ",port=%u", clnt->trans_opts.vsock.port);
+	}
+#endif
 	return 0;
 }
 
@@ -875,7 +887,7 @@ p9_fd_create_tcp(struct p9_client *client, struct fs_context *fc)
 	const char *addr = fc->source;
 	struct v9fs_context *ctx = fc->fs_private;
 	int err;
-	char port_str[6];
+	char port_str[11];
 	struct socket *csocket;
 	struct sockaddr_storage stor = { 0 };
 	struct p9_fd_opts opts;
@@ -966,6 +978,66 @@ p9_fd_create_unix(struct p9_client *client, struct fs_context *fc)
 	return p9_socket_open(client, csocket);
 }
 
+#if IS_ENABLED(CONFIG_NET_9P_VSOCK)
+static int
+p9_fd_create_vsock(struct p9_client *client, struct fs_context *fc)
+{
+	const char *addr = fc->source;
+	struct v9fs_context *ctx = fc->fs_private;
+	int err;
+	struct socket *csocket;
+	struct sockaddr_vm addr_vm = { 0 };
+	struct p9_fd_opts opts;
+	unsigned int cid;
+
+	/* opts are already parsed in context */
+	opts = ctx->fd_opts;
+
+	if (opts.privport) {
+		pr_err("%s (%d): privport not supported for vsock\n",
+		       __func__, task_pid_nr(current));
+		return -EOPNOTSUPP;
+	}
+
+	if (!addr)
+		return -EINVAL;
+
+	err = kstrtouint(addr, 10, &cid);
+	if (err < 0) {
+		pr_err("%s (%d): invalid CID: %s\n",
+		       __func__, task_pid_nr(current), addr);
+		return err;
+	}
+
+	csocket = NULL;
+
+	client->trans_opts.vsock.port = opts.port;
+	err = __sock_create(fc->net_ns, AF_VSOCK,
+			    SOCK_STREAM, 0, &csocket, 1);
+	if (err) {
+		pr_err("%s (%d): problem creating socket\n",
+		       __func__, task_pid_nr(current));
+		return err;
+	}
+
+	addr_vm.svm_family = AF_VSOCK;
+	addr_vm.svm_port = opts.port;
+	addr_vm.svm_cid = cid;
+
+	err = READ_ONCE(csocket->ops)->connect(csocket,
+					       (struct sockaddr_unsized *)&addr_vm,
+					       sizeof(addr_vm), 0);
+	if (err < 0) {
+		pr_err("%s (%d): problem connecting socket to %s:%u\n",
+		       __func__, task_pid_nr(current), addr, opts.port);
+		sock_release(csocket);
+		return err;
+	}
+
+	return p9_socket_open(client, csocket);
+}
+#endif /* CONFIG_NET_9P_VSOCK */
+
 static int
 p9_fd_create(struct p9_client *client, struct fs_context *fc)
 {
@@ -1036,6 +1108,23 @@ static struct p9_trans_module p9_fd_trans = {
 };
 MODULE_ALIAS_9P("fd");
 
+#if IS_ENABLED(CONFIG_NET_9P_VSOCK)
+static struct p9_trans_module p9_vsock_trans = {
+	.name = "vsock",
+	.maxsize = MAX_SOCK_BUF,
+	.def = false,
+	.supports_vmalloc = true,
+	.create = p9_fd_create_vsock,
+	.close = p9_fd_close,
+	.request = p9_fd_request,
+	.cancel = p9_fd_cancel,
+	.cancelled = p9_fd_cancelled,
+	.show_options = p9_fd_show_options,
+	.owner = THIS_MODULE,
+};
+MODULE_ALIAS_9P("vsock");
+#endif
+
 /**
  * p9_poll_workfn - poll worker thread
  * @work: work queue
@@ -1073,6 +1162,9 @@ static int __init p9_trans_fd_init(void)
 	v9fs_register_trans(&p9_tcp_trans);
 	v9fs_register_trans(&p9_unix_trans);
 	v9fs_register_trans(&p9_fd_trans);
+#if IS_ENABLED(CONFIG_NET_9P_VSOCK)
+	v9fs_register_trans(&p9_vsock_trans);
+#endif
 
 	return 0;
 }
@@ -1083,6 +1175,9 @@ static void __exit p9_trans_fd_exit(void)
 	v9fs_unregister_trans(&p9_tcp_trans);
 	v9fs_unregister_trans(&p9_unix_trans);
 	v9fs_unregister_trans(&p9_fd_trans);
+#if IS_ENABLED(CONFIG_NET_9P_VSOCK)
+	v9fs_unregister_trans(&p9_vsock_trans);
+#endif
 }
 
 module_init(p9_trans_fd_init);
-- 
2.47.1

Re: [PATCH v2] net/9p: add vsock transport

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 2914 bytes --]

On Wed, Jul 01, 2026 at 05:59:22PM +0200, skvarlamatus@gmail.com wrote:
> From: Matus Skvarla <skvarlamatus@gmail.com>
> 
> Add vsock as a transport option for 9P client connections. This
> allows mounting 9P filesystems over VM sockets without requiring TCP/IP
> networking or additional userspace tools.
> 
> The implementation extends trans_fd.c with vsock support, reusing the
> existing socket infrastructure. A new p9_fd_create_vsock() function
> handles vsock connection setup by parsing the CID from the mount source,
> creating an AF_VSOCK socket, and connecting to the specified endpoint.
> All other transport operations (close, request, cancel) use the shared
> fd transport implementation.
> 
> The privport mount option is not currently supported for vsock and
> returns -EOPNOTSUPP if specified.
> 
> Add CONFIG_NET_9P_VSOCK option that conditionally compiles vsock support
> into 9pnet_fd.ko. This follows the pattern where socket-based transports
> (TCP, Unix, vsock) share trans_fd.c, while specialized hardware transports
> (virtio, xen, rdma) have dedicated files.
> 
> Usage:
>   mount -t 9p -o trans=vsock[,port=<port>] <CID> /mnt/point
> 
> Signed-off-by: Matus Skvarla <skvarlamatus@gmail.com>
> ---
> Changes in v2:
>   - Add depends on NET_9P_FD to Kconfig
>   - Use #include <uapi/linux/vm_sockets.h>
>   - Reject privport option with -EOPNOTSUPP
>   - Use fc->net_ns instead of current->nsproxy->net_ns
>   - Widen vsock port to u32 to match sockaddr_vm.svm_port
>   - Widen p9_fd_opts.port to u32 and port_str buffer accordingly (in p9_fd_create_tcp())
>   - Add vsock to Documentation/filesystems/9p.rst
> Link to v1: https://lore.kernel.org/v9fs/20260527073447.86538-1-skvarlamatus@gmail.com/
> ---
>  Documentation/filesystems/9p.rst | 12 +++-
>  include/net/9p/client.h          | 10 +++-
>  net/9p/Kconfig                   | 10 ++++
>  net/9p/trans_fd.c                | 97 +++++++++++++++++++++++++++++++-
>  4 files changed, 124 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/filesystems/9p.rst b/Documentation/filesystems/9p.rst
> index 3f65db648db0..d1bac30b3c9d 100644
> --- a/Documentation/filesystems/9p.rst
> +++ b/Documentation/filesystems/9p.rst
> @@ -50,6 +50,14 @@ mount points. Each 9P export is seen by the client as a virtio device with an
>  associated "mount_tag" property. Available mount tags can be
>  seen by reading /sys/bus/virtio/drivers/9pnet_virtio/virtio<n>/mount_tag files.
>  
> +For server accessible over vsock (VM sockets)::
> +
> +	mount -t 9p -o trans=vsock <CID> /mnt/9

If you respin this patch, please add the port= option because it is
absent from this example and it could be missed if one doesn't read the
paragraph below carefully:

  mount -t 9p -o trans=vsock[,port=<port>] <CID> /mnt/9

Other than that:

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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Re: [PATCH v2] net/9p: add vsock transport

[Stefano Garzarella]

On Wed, Jul 01, 2026 at 05:59:22PM +0200, skvarlamatus@gmail.com wrote:
>From: Matus Skvarla <skvarlamatus@gmail.com>
>
>Add vsock as a transport option for 9P client connections. This
>allows mounting 9P filesystems over VM sockets without requiring TCP/IP
>networking or additional userspace tools.
>
>The implementation extends trans_fd.c with vsock support, reusing the
>existing socket infrastructure. A new p9_fd_create_vsock() function
>handles vsock connection setup by parsing the CID from the mount source,
>creating an AF_VSOCK socket, and connecting to the specified endpoint.
>All other transport operations (close, request, cancel) use the shared
>fd transport implementation.
>
>The privport mount option is not currently supported for vsock and
>returns -EOPNOTSUPP if specified.

Why not?

I'm no expert on 9p; it's just out of curiosity.

>
>Add CONFIG_NET_9P_VSOCK option that conditionally compiles vsock support
>into 9pnet_fd.ko. This follows the pattern where socket-based transports
>(TCP, Unix, vsock) share trans_fd.c, while specialized hardware transports
>(virtio, xen, rdma) have dedicated files.
>
>Usage:
>  mount -t 9p -o trans=vsock[,port=<port>] <CID> /mnt/point
>
>Signed-off-by: Matus Skvarla <skvarlamatus@gmail.com>
>---
>Changes in v2:
>  - Add depends on NET_9P_FD to Kconfig
>  - Use #include <uapi/linux/vm_sockets.h>
>  - Reject privport option with -EOPNOTSUPP
>  - Use fc->net_ns instead of current->nsproxy->net_ns
>  - Widen vsock port to u32 to match sockaddr_vm.svm_port
>  - Widen p9_fd_opts.port to u32 and port_str buffer accordingly (in p9_fd_create_tcp())
>  - Add vsock to Documentation/filesystems/9p.rst
>Link to v1: https://lore.kernel.org/v9fs/20260527073447.86538-1-skvarlamatus@gmail.com/
>---
> Documentation/filesystems/9p.rst | 12 +++-
> include/net/9p/client.h          | 10 +++-
> net/9p/Kconfig                   | 10 ++++
> net/9p/trans_fd.c                | 97 +++++++++++++++++++++++++++++++-
> 4 files changed, 124 insertions(+), 5 deletions(-)
>
>diff --git a/Documentation/filesystems/9p.rst b/Documentation/filesystems/9p.rst
>index 3f65db648db0..d1bac30b3c9d 100644
>--- a/Documentation/filesystems/9p.rst
>+++ b/Documentation/filesystems/9p.rst
>@@ -50,6 +50,14 @@ mount points. Each 9P export is seen by the client as a virtio device with an
> associated "mount_tag" property. Available mount tags can be
> seen by reading /sys/bus/virtio/drivers/9pnet_virtio/virtio<n>/mount_tag files.
>
>+For server accessible over vsock (VM sockets)::
>+
>+	mount -t 9p -o trans=vsock <CID> /mnt/9
>+
>+where CID is the vsock Context ID of the remote endpoint (2 for host from
>+a guest VM, 3+ for guest VMs from the host). The port to connect to can be
>+specified with the port option (default: 564).
>+
> USBG Usage
> ==========
>
>@@ -115,8 +123,10 @@ Options
> 			========  ============================================
> 			unix 	  specifying a named pipe mount point
> 			tcp	  specifying a normal TCP/IP connection
>-			fd   	  used passed file descriptors for connection
>+			fd   	  uses passed file descriptors for connection

nit: unrelated change?

The rest LGTM, so for vsock point of view:

Acked-by: Stefano Garzarella <sgarzare@redhat.com>

Re: [PATCH v2] net/9p: add vsock transport

[Dominique Martinet]

Matúš Škvarla wrote on Fri, Jul 03, 2026 at 02:31:41PM +0200:
> pi 3. 7. 2026 o 7:41 Stefan Hajnoczi <stefanha@redhat.com> napísal(a):
> 
> > If you respin this patch, please add the port= option because it is
> > absent from this example and it could be missed if one doesn't read the
> > paragraph below carefully:
> >
> >   mount -t 9p -o trans=vsock[,port=<port>] <CID> /mnt/9
> >
> 
> My first idea was to have it there, but I removed it before submitting
> as I thought it would be redundant. No problem with having the port
> option specified in the example. I'll submit a v3 with this change.

If that's the only change required I can do it when applying, don't
bother with a v3 just yet

(didn't check the patch itself yet, will probably take a couple of
weeks...)
-- 
Dominique Martinet | Asmadeus

Re: [PATCH v2] net/9p: add vsock transport

[Matus Skvarla]

On Fri, Jul 03, 2026 at 05:41 +0000, stefanha@redhat.com wrote:
> If you respin this patch, please add the port= option because it is
> absent from this example and it could be missed if one doesn't read the
> paragraph below carefully:
>
>	mount -t 9p -o trans=vsock[,port=<port>] <CID> /mnt/9

My first idea was to have it there, but I removed it before submitting
as I thought it would be redundant. No problem with having the port
option specified in the example. I'll submit a v3 with this change.

--
Matus

Re: [PATCH v2] net/9p: add vsock transport

[skvarlamatus]

On Fri, Jul 03, 2026 at 09:12 +0000, sgarzare@redhat.com wrote:
> >The privport mount option is not currently supported for vsock and
> >returns -EOPNOTSUPP if specified.
>
> Why not?

The existing p9_bind_privport() implementation is TCP-specific
(uses inet_pton and struct sockaddr_in), so it cannot be reused
for vsock directly. Per Stefan Hajnoczi's suggestion, I opted to
reject the option cleanly for now rather than leave it silently
ignored. Support can be added in a follow-up if needed.

> >-			fd   	  used passed file descriptors for connection
> >+			fd   	  uses passed file descriptors for connection
>
> nit: unrelated change?

I noticed it as I was adding the vsock line below, but you're right,
it's not related, so I'll drop it in v3.

> The rest LGTM, so for vsock point of view:
>
> Acked-by: Stefano Garzarella <sgarzare@redhat.com>

Thank you!

--
Matus

Re: [PATCH v2] net/9p: add vsock transport

[Stefano Garzarella]

On Fri, Jul 03, 2026 at 04:31:25PM +0200, skvarlamatus@gmail.com wrote:
>On Fri, Jul 03, 2026 at 09:12 +0000, sgarzare@redhat.com wrote:
>> >The privport mount option is not currently supported for vsock and
>> >returns -EOPNOTSUPP if specified.
>>
>> Why not?
>
>The existing p9_bind_privport() implementation is TCP-specific
>(uses inet_pton and struct sockaddr_in), so it cannot be reused
>for vsock directly. Per Stefan Hajnoczi's suggestion, I opted to
>reject the option cleanly for now rather than leave it silently

Ah, I see now. Sorry, I didn't look at the v1.

>ignored. Support can be added in a follow-up if needed.

I'd add a comment in the code to explain that, maybe with a TODO label.  
Because initially I thought it was not applicable to AF_VSOCK sockets 
for some reason related to he socket family.

Thanks,
Stefano

Re: [PATCH v2] net/9p: add vsock transport

[Matus Skvarla]

On Fri, Jul 03, 2026 at 05:41 +0000, stefanha@redhat.com wrote:
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

Thanks for the review!

--
Matus

Re: [PATCH v2] net/9p: add vsock transport

[skvarlamatus]

On Fri, Jul 03, 2026 at 14:16 +0000, asmadeus@codewreck.org wrote:
> If that's the only change required I can do it when applying, don't
> bother with a v3 just yet

No problem Dominique, there are a couple more small changes, not only
this one. I'll submit the v3 next week (probably). But there shouldn't
be any more significant changes to the code.

--
Matus