On Mon, Mar 02, 2026 at 04:48:33PM +0100, Alexander Graf wrote:

On 02.03.26 13:06, Stefano Garzarella wrote:

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CCing Bryan, Vishnu, and Broadcom list.

On Mon, Mar 02, 2026 at 12:47:05PM +0100, Stefano Garzarella wrote:

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Please target net-next tree for this new feature.

On Mon, Mar 02, 2026 at 10:41:38AM +0000, Alexander Graf wrote:

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Vsock maintains a single CID number space which can be used to
communicate to the host (G2H) or to a child-VM (H2G). The current logic
trivially assumes that G2H is only relevant for CID <= 2 because these
target the hypervisor.  However, in environments like Nitro
Enclaves, an
instance that hosts vhost_vsock powered VMs may still want to
communicate
to Enclaves that are reachable at higher CIDs through virtio-vsock-pci.

That means that for CID > 2, we really want an overlay. By default, all
CIDs are owned by the hypervisor. But if vhost registers a CID,
it takes
precedence.  Implement that logic. Vhost already knows which CIDs it
supports anyway.

With this logic, I can run a Nitro Enclave as well as a nested VM with
vhost-vsock support in parallel, with the parent instance able to
communicate to both simultaneously.

I honestly don't understand why VMADDR_FLAG_TO_HOST (added
specifically for Nitro IIRC) isn't enough for this scenario and we
have to add this change.  Can you elaborate a bit more about the
relationship between this change and VMADDR_FLAG_TO_HOST we added?

The main problem I have with VMADDR_FLAG_TO_HOST for connect() is that it
punts the complexity to the user. Instead of a single CID address space, you
now effectively create 2 spaces: One for TO_HOST (needs a flag) and one for
TO_GUEST (no flag). But every user space tool needs to learn about this
flag. That may work for super special-case applications. But propagating
that all the way into socat, iperf, etc etc? It's just creating friction.

IMHO the most natural experience is to have a single CID space, potentially
manually segmented by launching VMs of one kind within a certain range.

At the end of the day, the host vs guest problem is super similar to a
routing table.

If this is what's desired, some bits could be stolen from the CID
to specify the destination type. Would that address the issue?
Just a thought.

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Signed-off-by: Alexander Graf <graf@amazon.com>
---
drivers/vhost/vsock.c    | 11 +++++++++++
include/net/af_vsock.h   |  3 +++
net/vmw_vsock/af_vsock.c |  3 +++
3 files changed, 17 insertions(+)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index 054f7a718f50..223da817e305 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c

@@ -91,6 +91,16 @@ static struct vhost_vsock

*vhost_vsock_get(u32 guest_cid, struct net *net)
    return NULL;
}

+static bool vhost_transport_has_cid(u32 cid)
+{
+    bool found;
+
+    rcu_read_lock();
+    found = vhost_vsock_get(cid) != NULL;

We recently added namespaces support that changed vhost_vsock_get()
params. This is also in net tree now and in Linus' tree, so not sure
where this patch is based, but this needs to be rebased since it is
not building:

../drivers/vhost/vsock.c: In function ‘vhost_transport_has_cid’:
../drivers/vhost/vsock.c:99:17: error: too few arguments to function
‘vhost_vsock_get’; expected 2, have 1
  99 |         found = vhost_vsock_get(cid) != NULL;
     |                 ^~~~~~~~~~~~~~~
../drivers/vhost/vsock.c:74:28: note: declared here
  74 | static struct vhost_vsock *vhost_vsock_get(u32 guest_cid,
struct net *net)
     |

D'oh. Sorry, I built this on 6.19 and only realized after the send that
namespace support got in. Will fix up for v2.

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+    rcu_read_unlock();
+    return found;
+}
+
static void
vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
                struct vhost_virtqueue *vq)

@@ -424,6 +434,7 @@ static struct virtio_transport vhost_transport = {

        .module                   = THIS_MODULE,

        .get_local_cid            = vhost_transport_get_local_cid,
+        .has_cid                  = vhost_transport_has_cid,

        .init                     = virtio_transport_do_socket_init,
        .destruct                 = virtio_transport_destruct,

diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
index 533d8e75f7bb..4cdcb72f9765 100644
--- a/include/net/af_vsock.h
+++ b/include/net/af_vsock.h

@@ -179,6 +179,9 @@ struct vsock_transport {

    /* Addressing. */
    u32 (*get_local_cid)(void);

+    /* Check if this transport serves a specific remote CID. */
+    bool (*has_cid)(u32 cid);

What about "has_remote_cid" ?

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+
    /* Read a single skb */
    int (*read_skb)(struct vsock_sock *, skb_read_actor_t);

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 2f7d94d682cb..8b34b264b246 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c

@@ -584,6 +584,9 @@ int vsock_assign_transport(struct vsock_sock

*vsk, struct vsock_sock *psk)
        else if (remote_cid <= VMADDR_CID_HOST || !transport_h2g ||
             (remote_flags & VMADDR_FLAG_TO_HOST))
            new_transport = transport_g2h;
+        else if (transport_h2g->has_cid &&
+             !transport_h2g->has_cid(remote_cid))
+            new_transport = transport_g2h;

We should update the comment on top of this fuction, and maybe also
try to support the other H2G transport (i.e. VMCI).

@Bryan @Vishnu can the new has_cid()/has_remote_cid() be supported
by VMCI too?

Oops, I forgot to CC them, now they should be in copy.

Ack. I can also take a quick look if it's trivial to add.


Alex





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