Date:	Fri, 13 Mar 2009 10:04:50 +0100
From:	Andreas Herrmann <andreas.herrmann3@....com>
To:	Ingo Molnar <mingo@...e.hu>
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

On Fri, Mar 13, 2009 at 02:58:56AM +0100, Ingo Molnar wrote:
> 
> * Andreas Herrmann <andreas.herrmann3@....com> wrote:
> 
> > Impact: bug fix + BIOS workaround
> 
> > Change to previous version:
> > I slightly modified the log message (e.g. addition of FW_WARN).
> > 
> > Please consider to apply this patch for .29.
> 
> i've applied it to tip:x86/mtrr, thanks Andreas.
> 
> I've add a -stable backport tag - so if it's problem-free it 
> should show up in .29.1.

That should suffice.

> It is not completely clear what the impact of this fix is. What 
> types of problems are such incoherent MTRR settings causing in 
> practice?

I admit the commit message is not that explanatory ...

(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).

> Boot hang? S2RAM failures? Performance problems?

for (1) S2RAM and S2DISK failures.
for (2) boot hang

> Without knowing the exact impact we cannot apply it this late in 
> the .29.0 cycle - and MTRR code change are dangerous in any case 
> so even if we knew the exact scope and impact we'd probably not 
> do it in .29.

Fine with me (although I think that it's safest not to touch the two
bits at all from the OS as we don't know what the BIOS wants to do
with them).


Regards,

Andreas

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