linux-kernel - Re: [PATCH 4/8] x86 boot: allow overlapping ebda and efi memmap memory ranges

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Date:	Mon, 16 Jun 2008 12:37:21 -0500
From:	Paul Jackson <pj@....com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	ying.huang@...el.com, mingo@...e.hu, tglx@...utronix.de,
	yhlu.kernel@...il.com, steiner@....com, travis@....com,
	linux-kernel@...r.kernel.org, andi@...stfloor.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH 4/8] x86 boot: allow overlapping ebda and efi memmap
 memory ranges

hpa, replying to pj:
> >> Would you recommend doing this with code in arch/x86/kernel/head.c,
> >> that did not invoke reserve_ebda_region() if efi_enabled was set?
> 
> I disagree with it

Ok - that's clear.

So it would seem that I am getting conflicting advice.

One person recommends code that only makes this safety reservation of
the ebda region in the non-EFI case:

    if (!efi_enabled)
	reserve_ebda_region();

and the other recommends code that always makes this safety reservation,
and that handles the possible resulting overlap with the EFI memmap:

    if (!range_in_ebda_area(pmap, pmap + memmap.nr_map * memmap.desc_size))
	reserve_early(pmap, pmap + memmap.nr_map * memmap.desc_size,
	    "EFI memmap");

(The above code should be adjusted in light of Yinghai's suggestion
that it handle partial overlap.)

The resolution of this conflict might be easy, however.

I will readily accept that there exist some 'classic' PCs for which
we need to reserve a 'safe' ebda area.

The question to me is this.  Are there PCs which (1) need such a safety
reservation of an ebda area -and- (2) boot with EFI enabled?  I am not
asking if there -could- be (in the abstract, there certainly is no law
of government or physics prohibiting such).  Rather I am asking as a
practical matter if there is, or is likely to be, such PCs "in the wild."

The safety reservation of this ebda area is a hack.  As hacks go, it is
a rather gentle hack, but still it is a hack.  As such, it is to be
avoided unless there is a practical need.  Some non-efi old PCs have that
need - no debate there.

Should we perpeturate this (gentle) hack for EFI systems as well?

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@....com> 1.940.382.4214
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