linux-kernel - Re: [Xen-devel] [PATCH] kernel-parameters: document earlycon=xenboot

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Date:	Tue, 5 Apr 2016 18:16:35 -0700 (PDT)
From:	Stefano Stabellini <sstabellini@...nel.org>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
cc:	Chris Patterson <cjp256@...il.com>, xen-devel@...ts.xenproject.org,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jonathan Corbet <corbet@....net>
Subject: Re: [Xen-devel] [PATCH] kernel-parameters: document
 earlycon=xenboot

On Tue, 5 Apr 2016, Konrad Rzeszutek Wilk wrote:
> On Tue, Apr 05, 2016 at 11:43:36AM -0400, Chris Patterson wrote:
> > On Tue, Apr 5, 2016 at 9:49 AM, Konrad Rzeszutek Wilk
> > <konrad.wilk@...cle.com> wrote:
> > > On Mon, Apr 04, 2016 at 10:48:11PM -0400, Chris Patterson wrote:
> > >> Add earlycon=xenboot option to Documentation/kernel-parameters.txt.
> > >>
> > >
> > > But it is not true.
> > >
> > > I tried this on x86: "earlycon=xenboot console=tty0"

Was it a PV guest? Do you have SERIAL_EARLYCON enabled in your kconfig?
Does Xen allow DomU debug output via HYPERVISOR_console_io in your config?

> > I can investigate further, but hopefully Stefano can chime in more
> > definitively on what is supported.
> > 
> > I used "earlycon=xenboot console=hvc0" with success for dom0 on my
> > ARM64 platform.
> 
> Perhaps the documentation should also have [ARM], like some of the
> other ones?
> 

I don't recall whether I actually tested earlycon on x86, but looking at
the code it should work, at least for PV guests (you need to be able to
issue hypercalls and HVM guests cannot do that until they map the
hypercall page). 

In any case given that x86 has earlyprintk, specifying
xenboot as only available for ARM and ARM64 is also OK for me, but I
would prefer to keep it arch-independent given that earlycon is.