linux-kernel - Re: [RFC] use realmode code to reserve end-of-conventional-memory to 1MB

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Date:	Wed, 5 Mar 2008 12:59:31 -0300
From:	Eduardo Habkost <ehabkost@...hat.com>
To:	Ian Campbell <ijc@...lion.org.uk>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Alexander van Heukelum <heukelum@...lshack.com>,
	Ingo Molnar <mingo@...e.hu>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	Andi Kleen <ak@...e.de>, Thomas Gleixner <tglx@...utronix.de>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Mark McLoughlin <markmc@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] use realmode code to reserve end-of-conventional-memory
	to 1MB

On Thu, Feb 28, 2008 at 11:16:49PM +0000, Ian Campbell wrote:
> 
> The patch below seems like the right thing to do. It certainly boots in
> a domU without the DMI problem (without any of the other related patches
> such as Alexander's).
> 
> However ddcprobe hangs when run -- need to investigate some more, vm86
> in general works ok (i.e. my vm86 test code passes).
> 
> BTW Jeremy, the kernel doesn't use XENMEM_memory_map -- any reason other
> than it not being useful at the time? These days the tools can push an
> arbitrary e820 down for a guest which might be useful to support,
> although nothing interesting is done with it today.
> 
> Ian.
> 
> x86/xen: Construct e820 map with a hole between 640K-1M.
> 
> Signed-off-by: Ian Campbell <ijc@...lion.org.uk>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Jeremy Fitzhardinge <jeremy@...p.org>
> ---
>  arch/x86/xen/setup.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
> index 3bad477..2341492 100644
> --- a/arch/x86/xen/setup.c
> +++ b/arch/x86/xen/setup.c
> @@ -38,7 +38,8 @@ char * __init xen_memory_setup(void)
>  	unsigned long max_pfn = xen_start_info->nr_pages;
>  
>  	e820.nr_map = 0;
> -	add_memory_region(0, PFN_PHYS(max_pfn), E820_RAM);
> +	add_memory_region(0, LOWMEMSIZE(), E820_RAM);
> +	add_memory_region(HIGH_MEMORY, PFN_PHYS(max_pfn)-HIGH_MEMORY, E820_RAM);

Won't this waste 300+ KB of Perfectly Good RAM? Or I understood it
incorrectly?

I am aware that it would take more work to tell all kernel code that it
shouldn't look for BIOS data on this region when running as a domU guest,
but it seems that it would be a better solution.

-- 
Eduardo
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