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

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Date:   Wed, 23 Nov 2022 11:36:19 +0100
From:   Amit Shah <amit@...radead.org>
To:     Cédric Le Goater <clg@...d.org>,
        Amit Shah <amit@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, Thomas Huth <thuth@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH v2] virtio_console: Introduce an ID allocator for
 virtual console numbers

On Wed, 2022-11-23 at 11:17 +0100, Cédric Le Goater wrote:
> Hello Amit,
> 
> On 11/22/22 18:03, Amit Shah wrote:
> > 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...
> 
> 'next_vtermno' was introduced by d8a02bd58ab6 ("virtio: console:
> remove global var") to differentiate the underlying kernel hvc console
> associated with each virtio console port. Some drivers, like XEN,
> simply use a magic/cookie number for instance.
> 
> This number is not related to the virtio specs. It is not exposed to
> QEMU nor the guest (a part from debugfs). It's an internal identifier
> related to the implementation in the kernel. I don't understand how
> this could break compatibility. The change even keeps the allocated
> range the same in case some assumption is made on vtermno 0. Am I
> missing something ?

No, you're right about this being kernel-internal -- just that it's
used with hvc instead of qemu like I mentioned.

I think this is the right change; just want to confirm hvc didn't get
confused.

> 
> In the virtio console driver case, we could also generate a unique
> number from the tuple { virtio device index, virtio console port }.
> The ID allocator approach is simpler.

I think the bug is that we don't increment the vtermno today in all
places that we should; but this patch solves it too - I don't mind
adding the extra ida bits.