Subject: [PATCH] vsock: Enable H2G override

From: Alexander Graf <[email-masked]>
Date: Mon, 2 Mar 2026 10:41:38 +0000

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.

Signed-off-by: Alexander Graf <[email-masked]>
---
 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;
+	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);
+
 	/* 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;
 		else
 			new_transport = transport_h2g;
 		break;
-- 
2.47.1

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