On 10.03.26 10:26, Paolo Abeni wrote:

Hi,

On 3/10/26 10:18 AM, Paolo Abeni wrote:

quoted

This is an AI-generated review of your patch. The human sending this
email has considered the AI review valid, or at least plausible.

For local reproduction steps see https://netdev-ai.bots.linux.dev/ai-local.html
---

quoted

@@ -581,11 +585,21 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
      case SOCK_SEQPACKET:
              if (vsock_use_local_transport(remote_cid))
                      new_transport = transport_local;
-            else if (remote_cid <= VMADDR_CID_HOST || !transport_h2g ||
+            else if (remote_cid <= VMADDR_CID_HOST ||
                       (remote_flags & VMADDR_FLAG_TO_HOST))
                      new_transport = transport_g2h;
-            else
+            else if (transport_h2g &&
+                     (!transport_h2g->has_remote_cid ||
+                      transport_h2g->has_remote_cid(vsk, remote_cid)))
+                    new_transport = transport_h2g;
+            else if (sock_net(sk)->vsock.g2h_fallback &&
+                     transport_g2h && transport_g2h->has_remote_cid &&
+                     transport_g2h->has_remote_cid(vsk, remote_cid)) {
+                    vsk->remote_addr.svm_flags |= VMADDR_FLAG_TO_HOST;
+                    new_transport = transport_g2h;
+            } else {
                      new_transport = transport_h2g;
+            }
              break;

The old code had `!transport_h2g` in the second else-if condition at line
-583, which routed all CID > VMADDR_CID_HOST traffic to transport_g2h when
no H2G transport was loaded. The new code removes this check.

When transport_h2g is NULL (no H2G transport loaded) and the G2H transport
does not implement has_remote_cid (as is the case for VMCI and Hyper-V
transports), what happens?

The G2H fallback branch at line +596 requires
`transport_g2h->has_remote_cid` to be non-NULL. If it's NULL, the
short-circuit evaluation skips this branch and executes the final else at
line +601, setting `new_transport = transport_h2g` (which is NULL),
ultimately returning -ENODEV.

The commit message explicitly states the intent to preserve the existing
no-H2G fallback behavior. Does this implementation break it for G2H
transports that do not implement the new optional has_remote_cid callback?

This affects VMCI and Hyper-V environments: when no H2G transport is
loaded, CID > 2 connections that previously fell back to G2H now get
ENODEV.

Other comments from AI looked like nit picking to me, but the above one
looks relevant. I forwarded verbatim all the feedback for completeness.

Yes, this is on purpose. Instead of having bi-modality based on whether 
the vhost_vsock kernel module is loaded, we now clearly define that with 
the sysctl off, you get the same mode that "vhost_vsock is loaded" gave 
you before. The oddball case of "vhost_vsock is not loaded" that meant 
everything goes to the G2H transport and confused users is really not 
worth keeping IMHO. It's replaced by the auto-fallback logic.

Broadcom also mentioned that G2H for CID > 2 does not exist. IIUC 
Hyper-V also does not support CID > 2.


Alex




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