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

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Message-ID: <1349272182.650.150.camel@zakaz.uk.xensource.com>
Date:	Wed, 3 Oct 2012 14:49:42 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen: point xen_start_info to a dummy struct
 for PV on HVM guests

On Wed, 2012-10-03 at 14:37 +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)

But anyone who calls this before xen_start_info is setup is going to get
a bogus result, specifically in this case they will think they are domU
when in reality they are dom0 -- wouldn't it be better to fix those
callsites?

Perhaps turn this into a static inline with a BUG_ON(!xen_start_info) to
make catching these cases easier?

> 
> 
> 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>
> 
> 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;
> +struct start_info *xen_start_info = &_xen_start_info;
>  EXPORT_SYMBOL_GPL(xen_start_info);
>  
>  struct shared_info xen_dummy_shared_info;
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@...ts.xen.org
> http://lists.xen.org/xen-devel


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