linux-kernel - Re: [Xen-devel] [PATCH 5/9] ARM: Xen: fix initial build problems:

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Date:	Wed, 10 Oct 2012 17:01:26 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Ian Campbell <Ian.Campbell@...rix.com>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	Arnd Bergmann <arnd@...db.de>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Russell King <rmk+kernel@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Xen-devel] [PATCH 5/9] ARM: Xen: fix initial build problems:

On Wed, 10 Oct 2012, Ian Campbell wrote:
> On Wed, 2012-10-10 at 15:47 +0100, Stefano Stabellini wrote:
> > On Wed, 10 Oct 2012, Ian Campbell wrote:
> > > On Tue, 2012-10-09 at 19:21 +0100, Arnd Bergmann wrote:
> > > > On Tuesday 09 October 2012, Stefano Stabellini wrote:
> > > > > >  config XEN
> > > > > >       bool "Xen guest support on ARM (EXPERIMENTAL)"
> > > > > >       depends on EXPERIMENTAL && ARM && OF
> > > > > > +     depends on !CPU_V6
> > > > > >       help
> > > > > >         Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.
> > > > > 
> > > > > Considering that we rely on the virtualization extensions, this one can
> > > > > be:
> > > > > 
> > > > >     depends on CPU_V7
> > > > > 
> > > > > The rest looks fine. I can submit a second patch to change !CPU_V6 into
> > > > > CPU_V7 later, if you prefer.
> > > > 
> > > > CPU_V6 and CPU_V7 are not exclusive, I saw the problem when building a
> > > > combined kernel for both V6 and V7. The code also needs to depend on ARMv7
> > > > with virtualization extensions, but that is a different issue. We don't
> > > > actually have a configuration symbol for that yet, as far as I can tell.
> > > 
> > > I don't think the guest kernels (including dom0) need the extensions to
> > > run under Xen, they are only need by Xen itself. The guests should just
> > > see a relatively normal v7 processor. 
> > > 
> > > Stefano, does that sound right?
> > 
> > Keep in mind that we are using HVC to issue hypercalls, and HVC has been
> > introduced with the virtualization extensions, if I am not mistaken.
> 
> I think we can ignore that in this context since it is only used by bits
> which are already virtualisation aware -- i.e. you wouldn't want to add
> it to the Kconfig as a dependency for Xen.

Considering that ARM CPU_V* symbols are not mutually exclusive, if Linux
had a CPU_V7_VIRTEXT symbol I don't see why we shouldn't add it to our
list of dependencies. It is more accurate than CPU_V7 after all.

But I don't think that it is necessary to introduce one now only for
Xen, because like you said, we only need HVC and that is only used in
virtualization aware code.
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