linux-kernel - Re: [PATCH v2] x86: mtrr: don't modify RdDram/WrDram bits of fixed MTRRs

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Date:	Fri, 13 Mar 2009 10:21:43 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andreas Herrmann <andreas.herrmann3@....com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	trenn@...e.de, Yinghai Lu <yinghai@...nel.org>
Subject: Re: [PATCH v2] x86: mtrr: don't modify RdDram/WrDram bits of fixed
	MTRRs


* Andreas Herrmann <andreas.herrmann3@....com> wrote:

> (1) The patch modifies an old fix from Bernhard Kaindl to get
>     suspend/resume working on some Acer Laptops. Bernhard's patch
>     tried to sync RdMem/WrMem bits of fixed MTRR registers and that
>     helped on those old Laptops. (Don't ask me why -- can't test it
>     myself). But this old problem was not the motivation for the
>     patch. (See http://lkml.org/lkml/2007/4/3/110)
> 
> (2) The more important effect is to fix issues on some more current systems.
> 
>     On those systems Linux panics or just freezes, see
> 
>     http://bugzilla.kernel.org/show_bug.cgi?id=11541
>     (and also duplicates of this bug:
>     http://bugzilla.kernel.org/show_bug.cgi?id=11737
>     http://bugzilla.kernel.org/show_bug.cgi?id=11714)
> 
>     The affected systems boot only using acpi=ht, acpi=off or
>     when the kernel is built with CONFIG_MTRR=n.
> 
>     The acpi options prevent full enablement of ACPI.  Obviously when
>     ACPI is enabled the BIOS/SMM modfies RdMem/WrMem bits.  When
>     CONFIG_MTRR=y Linux also accesses and modifies those bits when it
>     needs to sync fixed-MTRRs across cores (Bernhard's fix, see (1)).
>     How do you synchronize that? You can't. As a consequence Linux
>     shouldn't touch those bits at all (Rationale are AMD's BKDGs which
>     recommend to clear the bit that makes RdMem/WrMem accessible).
>     This is the purpose of this patch. And (so far) this suffices to
>     fix (1) and (2).

thanks - that's excellent info. I've amended the commit log with 
this.

It's still .29.1 material due to the general riskiness of MTRR 
changes - but the merge window will open in 1-2 weeks so it's 
not a 3 months delay.

	Ingo
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