Re: [PATCH] vsock: Enable H2G override

[Alexander Graf <graf@amazon.com>]

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.


>>
>>>
>>> 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)
>>      |


D'oh. Sorry, I built this on 6.19 and only realized after the send that 
namespace support got in. Will fix up for v2.


>>
>>> +    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?
>
> Oops, I forgot to CC them, now they should be in copy.


Ack. I can also take a quick look if it's trivial to add.


Alex





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