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

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Date: Tue, 28 May 2024 18:38:24 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Stefano Garzarella <sgarzare@...hat.com>
Cc: Alexander Graf <graf@...zon.com>, 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 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? 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.

Paolo