On Tue, Mar 03, 2026 at 07:51:32AM +0100, Alexander Graf wrote:

On 02.03.26 20:52, Michael S. Tsirkin wrote:

quoted

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

quoted

On 02.03.26 13:06, Stefano Garzarella wrote:

quoted

CCing Bryan, Vishnu, and Broadcom list.

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

quoted

Please target net-next tree for this new feature.

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

quoted

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.

If we had thought of this from the beginning, yes. But now that everyone
thinks CID (guest) == CID (host), I believe this is no longer feasible.


Alex

I don't really insist, but just to point out that if we wanted to, we
could map multiple CIDs to host. Anyway.

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