[PATCH 0/2] Early printk support for virtio console devices.

[Will Deacon]

On Wed, May 01, 2013 at 07:53:49AM +0100, Anup Patel wrote:
> On Wed, May 1, 2013 at 7:37 AM, Rusty Russell <rusty at rustcorp.com.au> wrote:
> > Alexander Graf <agraf at suse.de> writes:
> >> There are not device specific registers in
> >> virtio-console. Virtio-console lives behind a virtio bus which doesn't
> >> know what these registers are.
> >
> > You're not going to make coherent arguments without reading that actual
> > patch we're discussing.  And you're going to just waste everyone else's
> > time.
> >
> >> Even if you shove it into config space,
> >
> > Which is what the patch does...
> >
> >> it'd be broken because config access happens without intercepts on
> >> some platforms.
> >
> > But people keep assuming it, which is one reason virtio_ccw switched
> > from passive lguest-style config to active.  Only lguest and the old kvm
> > virtio use it, and lguest can change.
> >
> > An emergency output is a reasonable idea, and this is a reasonable
> > implementation.  The question is practical: will it be used?  Because we
> > don't implement reasonable ideas which aren't going to be used.
> 
> We are already using it for Guest bring-up on APM ARM64 board using
> KVMTOOL and so far it has been really helpful in debugging KVM ARM64
> guest issues.
> 
> Also, this is a very useful debugging method for Guest bring-up on
> other architectures supported by KVM and other Hypervisors that
> support VirtIO.

FWIW: I've hacked together a small set of patches allowing us to use the
x86 ioport peripherals on arm/arm64 in kvmtool. This means you can use the
8250 for earlyprintk on the latter, and even as the main console if you
like since it has a device-tree binding.

I'll post the patches as an RFC to the kvm list.

Will