netdev - Re: How to implement message forwarding from one CID to another in vhost driver

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Date: Wed, 29 May 2024 10:04:52 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Paolo Bonzini <pbonzini@...hat.com>, Alexander Graf <graf@...zon.com>
Cc: Alexander Graf <agraf@...raf.de>, 
	Dorjoy Chowdhury <dorjoychy111@...il.com>, virtualization@...ts.linux.dev, kvm@...r.kernel.org, 
	netdev@...r.kernel.org, stefanha@...hat.com
Subject: Re: How to implement message forwarding from one CID to another in
 vhost driver

On Tue, May 28, 2024 at 06:38:24PM GMT, Paolo Bonzini wrote:
>On Tue, May 28, 2024 at 5:53 PM Stefano Garzarella <sgarzare@...hat.com> wrote:
>>
>> On Tue, May 28, 2024 at 05:49:32PM GMT, Paolo Bonzini wrote:
>> >On Tue, May 28, 2024 at 5:41 PM Stefano Garzarella <sgarzare@...hat.com> wrote:
>> >> >I think it's either that or implementing virtio-vsock in userspace
>> >> >(https://lore.kernel.org/qemu-devel/30baeb56-64d2-4ea3-8e53-6a5c50999979@redhat.com/,
>> >> >search for "To connect host<->guest").
>> >>
>> >> For in this case AF_VSOCK can't be used in the host, right?
>> >> So it's similar to vhost-user-vsock.
>> >
>> >Not sure if I understand but in this case QEMU knows which CIDs are
>> >forwarded to the host (either listen on vsock and connect to the host,
>> >or vice versa), so there is no kernel and no VMADDR_FLAG_TO_HOST
>> >involved.
>>
>> I meant that the application in the host that wants to connect to the
>> guest cannot use AF_VSOCK in the host, but must use the one where QEMU
>> is listening (e.g. AF_INET, AF_UNIX), right?
>>
>> I think one of Alex's requirements was that the application in the host
>> continue to use AF_VSOCK as in their environment.
>
>Can the host use VMADDR_CID_LOCAL for host-to-host communication?

Yep!

>If
>so, the proposed "-object vsock-forward" syntax can connect to it and
>it should work as long as the application on the host does not assume
>that it is on CID 3.

Right, good point!
We can also support something similar in vhost-user-vsock, where instead 
of using AF_UNIX and firecracker's hybrid vsock, we can redirect 
everything to VMADDR_CID_LOCAL.

Alex what do you think? That would simplify things a lot to do.
The only difference is that the application in the host has to talk to 
VMADDR_CID_LOCAL (1).

Stefano