From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tim Deegan <tim@xen.org>
Subject: Re: [PATCH v2] x86: debugging code for testing 16Tb
	support on smaller memory systems
Date: Thu, 24 Jan 2013 12:36:37 +0000
Message-ID: <20130124123637.GE18850@ocelot.phlegethon.org>
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To: Jan Beulich <JBeulich@suse.com>
Cc: xen-devel <xen-devel@lists.xen.org>
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At 12:23 +0000 on 24 Jan (1359030221), Jan Beulich wrote:
> >>> On 24.01.13 at 12:36, Tim Deegan <tim@xen.org> wrote:
> > At 14:26 +0000 on 23 Jan (1358951188), Jan Beulich wrote:
> >> --- a/xen/arch/x86/setup.c
> >> +++ b/xen/arch/x86/setup.c
> >> @@ -82,6 +82,11 @@ boolean_param("noapic", skip_ioapic_setu
> >>  s8 __read_mostly xen_cpuidle = -1;
> >>  boolean_param("cpuidle", xen_cpuidle);
> >>  
> >> +#ifndef NDEBUG
> >> +unsigned long __initdata highmem_start;
> >> +size_param("highmem-start", highmem_start);
> >> +#endif
> >> +
> >>  cpumask_t __read_mostly cpu_present_map;
> >>  
> >>  unsigned long __read_mostly xen_phys_start;
> >> @@ -787,6 +792,14 @@ void __init __start_xen(unsigned long mb
> >>      modules_headroom = bzimage_headroom(bootstrap_map(mod), mod->mod_end);
> >>      bootstrap_map(NULL);
> >>  
> >> +#ifndef highmem_start
> >> +    /* Don't allow split below 4Gb. */
> >> +    if ( highmem_start < GB(4) )
> >> +        highmem_start = 0;
> >> +    else /* align to L3 entry boundary */
> >> +        highmem_start &= ~((1UL << L3_PAGETABLE_SHIFT) - 1);
> >> +#endif
> > 
> > DYM #ifndef NDEBUG ?  I can see that checking for highmem_start being a
> > macro is strictly correct
> 
> I intended it to be that way, because there could be other uses
> for having the symbol #define-d/real.

Yes - but if it ever ends up being a #define _and_ user-settable, these
checks will silently disappear.  Since there's no indication in the
places where you might make it a #define that doing so will remove these
checks, I'd be inclined to leave it gated on NDEBUG so it's fail in an
obvious way.

Or add a #define CONFIG_HIGHMEM_START (default to == !NDEBUG), and gate
everything on that?

Tim.