linux-kernel - Re: Bug#584846: Detects only 64MB and fails to boot on Intel Green City board if e820 hooked by GRUB2

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Message-ID: <1276351120.14011.194.camel@localhost>
Date:	Sat, 12 Jun 2010 14:58:40 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Cc:	Josh Triplett <josh@...htriplett.org>, 584846@...s.debian.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Bug#584846: Detects only 64MB and fails to boot on Intel Green
 City board if e820 hooked by GRUB2

Josh Triplett reported this problem with memory sizing:

On Fri, 2010-06-11 at 23:03 -0700, Josh Triplett wrote:
> Package: linux-2.6
> Severity: normal
> 
> I managed to reproduce the problem using stock upstream kernels and
> defconfig, and with defconfig (and no initramfs) the kernel managed to
> use little enough memory that it booted successfully with <64MB of RAM.
> 
> Investigating, I found that Linux decided not to use e820, and instead
> decided to use the older BIOS function 0x88, which cannot report more
> than 64MB of RAM.
> 
> With some investigation and bisection, I managed to track the problem
> down to the following commit:
> 
> commit c549e71d073a6e9a4847497344db28a784061455
> Author: H. Peter Anvin <hpa@...or.com>
> Date:   Sat Mar 28 13:53:26 2009 -0700
> 
>     x86, setup: ACPI 3, BIOS workaround for E820-probing code
> 
>     Impact: ACPI 3 spec compliance, BIOS bug workaround
> 
>     The ACPI 3 spec added another field to the E820 buffer -- which is
>     backwards incompatible, since it contains a validity bit.
>     Furthermore, there has been at least one report of a BIOS which
>     assumes that the buffer it is pointed at is the same buffer as for the
>     previous E820 call.  Therefore, read the data into a temporary buffer
>     and copy the standard part of it if and only if the valid bit is set.
> 
>     Signed-off-by: H. Peter Anvin <hpa@...or.com>
> 
> 
> A kernel built from c549e71d073a6e9a4847497344db28a784061455 finds <64MB
> of RAM; a kernel built from c549e71d073a6e9a4847497344db28a784061455^
> successfully finds all 4GB of RAM.
> 
> Also note that newer upstream kernels, including v2.6.35-rc3, fail as
> well.  Since later kernels revert part of the above commit, the issue
> must lie with the parts of the commit not reverted.
> 
> And, again, I can reproduce this using the stock upstream GRUB2 1.98
> release built from source, by booting it from a USB key, and then
> booting the disk MBR via:
> 
> set root=(hd1)
> drivemap (hd1) (hd0)
> chainloader +1
> boot
> 
> 
> Nothing special about drivemap here; anything that uses grub's mmap
> module to reserve memory via e820 (GRUB_MACHINE_MEMORY_RESERVED) will
> cause grub to hook e820 and trigger this bug.  However, in stock grub,
> only drivemap does this.
> 
> - Josh Triplett
> 
> 
> 

-- 
Ben Hutchings
Once a job is fouled up, anything done to improve it makes it worse.

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