Re: [PATCH v2] virtio_console: Introduce an ID allocator for virtual console numbers

[Amit Shah]

On Mon, 2022-11-14 at 18:38 +0100, Cédric Le Goater wrote:
> When a virtio console port is initialized, it is registered as an hvc
> console using a virtual console number. If a KVM guest is started with
> multiple virtio console devices, the same vtermno (or virtual console
> number) can be used to allocate different hvc consoles, which leads to
> various communication problems later on.
> 
> This is also reported in debugfs :
> 
>   # grep vtermno /sys/kernel/debug/virtio-ports/*
>   /sys/kernel/debug/virtio-ports/vport1p1:console_vtermno: 1
>   /sys/kernel/debug/virtio-ports/vport2p1:console_vtermno: 1
>   /sys/kernel/debug/virtio-ports/vport3p1:console_vtermno: 2
>   /sys/kernel/debug/virtio-ports/vport4p1:console_vtermno: 3
> 
> Replace the next_vtermno global with an ID allocator and start the
> allocation at 1 as it is today. Also recycle IDs when a console port
> is removed.

When the original virtio_console module was written, it didn't have
support for multiple ports to be used this way.  So the oddity you're
seeing is left there deliberately: VMMs should not be instantiating
console ports this way.

I don't know if we should take in this change, but can you walk through
all combinations of new/old guest and new/old hypervisor and ensure
nothing's going to break -- and confirm with the spec this is still OK
to do?  It may not be a goal to still ensure launches of a new guest on
a very old (say) Centos5 guest still works -- but that was the point of
maintaining backward compat...


		Amit