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Message-ID: <6293fd7d415950979c55c3f09254a6f52eba708b.camel@infradead.org>
Subject: Re: [PATCH v2] virtio_console: Introduce an ID allocator for
 virtual console numbers
From:   Amit Shah <amit@infradead.org>
To:     =?ISO-8859-1?Q?C=E9dric?= Le Goater <clg@kaod.org>,
        Amit Shah <amit@kernel.org>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org
Date:   Tue, 22 Nov 2022 18:03:05 +0100
In-Reply-To: <20221114173811.1977367-1-clg@kaod.org>
References: <20221114173811.1977367-1-clg@kaod.org>
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On Mon, 2022-11-14 at 18:38 +0100, C=C3=A9dric 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.
>=20
> This is also reported in debugfs :
>=20
>   # 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
>=20
> 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