linux-kernel - Re: [PATCH] xen: point xen_start_info to a dummy struct for PV on HVM guests

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Message-ID: <alpine.DEB.2.02.1210031648330.29232@kaball.uk.xensource.com>
Date:	Wed, 3 Oct 2012 16:48:57 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	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>
Subject: Re: [PATCH] xen: point xen_start_info to a dummy struct for PV on
 HVM guests

On Wed, 3 Oct 2012, Konrad Rzeszutek Wilk wrote:
> On Wed, Oct 03, 2012 at 02:37:53PM +0100, Stefano Stabellini wrote:
> > PV on HVM guests don't have a start_info page mapped by Xen, so
> > xen_start_info is just NULL for them.
> > That is problem because other parts of the code expect xen_start_info to
> > point to something valid, for example xen_initial_domain() is defined as
> > follow:
> > 
> > #define xen_initial_domain()    (xen_domain() && \
> >                  xen_start_info->flags & SIF_INITDOMAIN)
> > 
> 
> .. introduced by commit 4c071ee5268f7234c3d084b6093bebccc28cdcba
> ("arm: initial Xen support)
> 
> > 
> > Allocate a dummy start_info struct and point xen_start_info to it, as we
> > do on ARM.
> > This is not going to change things for PV guests because
> > xen_start_info is set by arch/x86/xen/xen-head.S:startup_xen.
> > 
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@...citrix.com>
> 
> Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> > 
> > diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> > index bf788d3..5f242cb 100644
> > --- a/arch/x86/xen/enlighten.c
> > +++ b/arch/x86/xen/enlighten.c
> > @@ -96,7 +96,8 @@ EXPORT_SYMBOL(machine_to_phys_mapping);
> >  unsigned long  machine_to_phys_nr;
> >  EXPORT_SYMBOL(machine_to_phys_nr);
> >  
> > -struct start_info *xen_start_info;
> > +static struct start_info _xen_start_info;
> 
> And lets change that to
> 'xen_dummy_start_info' to keep in sync with the other dummy one.
> 
> And also add a commnt:
> 
> /*
>  * Since 'xen_initial_domain' dereferences the xen_start_info we need
>  * a dummy structure filled with zeros (for PVHVM guests which initialize
>  * this late). For PV guests we do not have to worry about this as the first
>  * few instructions (startup_xen) set it  properly.
>  */

I agree with all the changes, I'll repost soon

> > +struct start_info *xen_start_info = &_xen_start_info;
> >  EXPORT_SYMBOL_GPL(xen_start_info);
> >  
> >  struct shared_info xen_dummy_shared_info;
> 
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