linux-kernel - Re: [PATCH] reserve end-of-conventional-memory to 1MB on 32-bit v2

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Date:	Tue, 04 Mar 2008 11:44:42 +0000
From:	Mark McLoughlin <markmc@...hat.com>
To:	Alexander van Heukelum <heukelum@...lshack.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Ian Campbell <ijc@...lion.org.uk>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] reserve end-of-conventional-memory to 1MB on 32-bit v2

On Sat, 2008-03-01 at 17:09 +0100, Alexander van Heukelum wrote:
> This patch adds explicit detection of the EBDA and reservation
> of the rom and adapter address space 0xa0000-0x100000 to the
> i386 kernels. Before this patch, the EBDA size was hardcoded
> as 4Kb. Also, the reservation of the adapter range was done by
> modifying the e820 map which is now not necessary any longer,
> and that code is removed from copy_e820_map.
> 
> The amount of conventional memory and the start of the EBDA are
> detected by reading the BIOS data area directly. Paravirtual
> environments do not provide this area, so we bail out early
> in that case. They will just have to set up a correct memory
> map to start with.
> 
> Signed-off-by: Alexander van Heukelum <heukelum@...tmail.fm>
> 
> ---
> 
> Hi Ingo,
> 
> This is the second attempt at a i386-version of the ebda
> patch. I hope that one of the Xen people will be able to
> check that this does not break their setups, but I think
> it will be fine after their patch to exclude the 0x9f000-
> 0x100000 area explicitly in their setup.

Confirmed that with Ian's e820 map patch and your patch, Xen DomU boots
fine.

> diff --git a/arch/x86/kernel/setup_32.c b/arch/x86/kernel/setup_32.c
> index a1d7071..20e537b 100644
> --- a/arch/x86/kernel/setup_32.c
> +++ b/arch/x86/kernel/setup_32.c
> @@ -385,15 +385,60 @@ unsigned long __init find_max_low_pfn(void)
>  	return max_low_pfn;
>  }
>  
> +#define BIOS_EBDA_SEGMENT 0x40E
> +#define BIOS_LOWMEM_KILOBYTES 0x413
> +
>  /*
> - * workaround for Dell systems that neglect to reserve EBDA
> + * The BIOS places the EBDA/XBDA at the top of conventional
> + * memory, and usually decreases the reported amount of
> + * conventional memory (int 0x12) too. This also contains a
> + * workaround for Dell systems that neglect to reserve EBDA.
> + * The same workaround also avoids a problem with the AMD768MPX
> + * chipset: reserve a page before VGA to prevent PCI prefetch
> + * into it (errata #56). Usually the page is reserved anyways,
> + * unless you have no PS/2 mouse plugged in.
>   */
>  static void __init reserve_ebda_region(void)
>  {
> -	unsigned int addr;
> -	addr = get_bios_ebda();
> -	if (addr)
> -		reserve_bootmem(addr, PAGE_SIZE, BOOTMEM_DEFAULT);
> +	unsigned int lowmem, ebda_addr;
> +
> +	/* To determine the position of the EBDA and the */
> +	/* end of conventional memory, we need to look at */
> +	/* the BIOS data area. In a paravirtual environment */
> +	/* that area is absent. We'll just have to assume */
> +	/* that the paravirt case can handle memory setup */
> +	/* correctly, without our help. */
> +#ifdef CONFIG_PARAVIRT
> +	if ((boot_params.hdr.version >= 0x207) &&
> +			(boot_params.hdr.hardware_subarch != 0)) {
> +		return;
> +	}
> +#endif

This is a bit magic, is it worth splitting it out as something like
is_paravirt_environment() ?

Cheers,
Mark.

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