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

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Message-ID: <20121003134323.GA31270@phenom.dumpdata.com>
Date:	Wed, 3 Oct 2012 09:43:23 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xen: point xen_start_info to a dummy struct for PV on
 HVM guests

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.
 */
> +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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