Re: [PATCH] vsock: Enable H2G override
From: Stefano Garzarella <sgarzare@redhat.com>
Date: 2026-03-02 11:47:17
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kvm, lkml, virtualization
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?
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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)
|
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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? 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 Amazon Web Services Development Center Germany GmbH Tamara-Danz-Str. 13 10243 Berlin Geschaeftsfuehrung: Christof Hellmis, Andreas Stieger Eingetragen am Amtsgericht Charlottenburg unter HRB 257764 B Sitz: Berlin Ust-ID: DE 365 538 597