From: Ben Hutchings on 12 Jun 2010 10:10
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(a)zytor.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(a)zytor.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
> 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
Once a job is fouled up, anything done to improve it makes it worse.