linux-kernel - Re: Corrupted low memory in v3.9+

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Date:	Mon, 11 Nov 2013 15:31:23 -0800
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Olof Johansson <olof@...om.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Corrupted low memory in v3.9+

On 11/11/2013 03:35 AM, Ingo Molnar wrote:
> 
> If we reserve everything in low memory, all the time (which I very much 
> argue we should do) then the checker becomes a no-op and can be removed.
> 

Oops!  I had misunderstood how the checker worked -- I thought it
checked the *reserved* memory, but it in fact reserves memory
*independently* and then checks it.

The problem is fundamentally that setup_bios_corruption_check(); is
called too early in setup_arch() -- quite possibly due to other code
movement around it:

#ifdef CONFIG_X86_CHECK_BIOS_CORRUPTION
	setup_bios_corruption_check();
#endif

	reserve_real_mode();

	trim_platform_memory_ranges();
	trim_low_memory_range();

	init_mem_mapping();

setup_bios_corruption_check() should presumably be called between
trim_low_memory_range() and init_mem_mapping().  I'm actually surprised
that we don't trip on this *all the time* since the realmode trampoline
falls in this area...

At the same time, we should change the default for
CONFIG_X86_RESERVE_LOW to 640, and perhaps move it under EXPERT.

What do you guys think?

	-hpa

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