Re: [PATCH] vsock: Enable H2G override

[Stefano Garzarella <sgarzare@redhat.com>]

Please target net-next tree for this new feature.

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

>
>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)
       |

>+	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" ?

>+
> 	/* 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?



I have a question: until now, transport assignment was based simply on 
analyzing local socket information (vsk->remote_addr), but now we are 
also adding the status of other components (e.g., VMs that have started 
and registered the CID in vhost-vsock).

Could this produce strange behavior?
For example, two sockets with the same remote_addr communicate with the 
host or with the guest depending on whether or not the VM existed when 
they were created.

Thanks,
Stefano

> 		else
> 			new_transport = transport_h2g;
> 		break;
>-- 
>2.47.1
>
>
>
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