linux-kernel - Re: [PATCH 0/2] Early printk support for virtio console devices.

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Message-ID: <20130501144622.GA24055@mudshark.cambridge.arm.com>
Date:	Wed, 1 May 2013 15:46:22 +0100
From:	Will Deacon <will.deacon@....com>
To:	Anup Patel <anup@...infault.org>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"patches@...aro.org" <patches@...aro.org>,
	"linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
	Alexander Graf <agraf@...e.de>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Pranavkumar Sawargaonkar <pranavkumar@...aro.org>
Subject: Re: [PATCH 0/2] Early printk support for virtio console devices.

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@...tcorp.com.au> wrote:
> > Alexander Graf <agraf@...e.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
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