On 02.03.26 20:52, Michael S. Tsirkin wrote:
On Mon, Mar 02, 2026 at 04:48:33PM +0100, Alexander Graf wrote:
On 02.03.26 13:06, Stefano Garzarella wrote:
CCing Bryan, Vishnu, and Broadcom list.
On Mon, Mar 02, 2026 at 12:47:05PM +0100, Stefano Garzarella wrote:
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?
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
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