linux-kernel - Re: [PATCH] arm: introduce a DTS for Xen unprivileged virtual machines

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Message-Id: <201209201327.45802.arnd@arndb.de>
Date:	Thu, 20 Sep 2012 13:27:45 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Ian Campbell <Ian.Campbell@...rix.com>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Pawel Moll <pawel.moll@....com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>
Subject: Re: [PATCH] arm: introduce a DTS for Xen unprivileged virtual machines

On Thursday 20 September 2012, Ian Campbell wrote:
> On Thu, 2012-09-20 at 12:56 +0100, Stefano Stabellini wrote:
> > On Thu, 20 Sep 2012, Ian Campbell wrote:
> > > On Wed, 2012-09-19 at 18:44 +0100, Stefano Stabellini wrote:

> > > > + compatible = "xen,xenvm-4.2", "arm,vexpress";
> > > 
> > > Is this second compatible thing actually true? We don't actually emulate
> > > much (anything?) of what would be on a real vexpress motherboard.
> > > 
> > > "arm,vexpress" is used only in v2m.c and I don't think we want the
> > > majority of that -- we don't provide any of the peripherals which it
> > > registers.
> > > 
> > > I think the only things we might want out of that lot are the arch timer
> > > and perhaps the uart0 (as a debug port).
> > > 
> > > I suspect we should have our own xen machine .c.
> > > 
> > > [...]
> > 
> > It is true that we are "arm,vexpress" compatible at the moment.
> 
> But we aren't, we don't emulate 90%+ of the actual hardware which
> vexpress compatibility would actually imply.
> 
> Look in arch/arm/mach-vexpress/v2m.c, which is the only thing keyed off
> this compat value -- it's full of stuff which we don't (and aren't going
> to) implement.

It's not much different in the end, but I think I'd rather make the
compatible list in the device tree "xen,xenvm-4.2", "xen,xenvm" without
listing "arm,vexpress", but then adding "xen,xenvm" to the list of
compatible devices in the vexpress kernel code.

The main difference is that if we decide to separate out the Linux
code for Xen and vexpress later into distinct ports, we have the
option to do that. vexpress will support multiplatform configurations
in 3.7 anyway, so the idea of making all virtual platforms part of
vexpress in order to be able to boot the same kernel on them is not
all that important any more.

	Arnd
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