linux-kernel - Re: [PATCH] trim memory not covered by WB MTRRs

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Message-ID: <20070705121203.GB5167@ucw.cz>
Date:	Thu, 5 Jul 2007 12:12:03 +0000
From:	Pavel Machek <pavel@....cz>
To:	Jesse Barnes <jesse.barnes@...el.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	Andi Kleen <andi@...stfloor.org>,
	Justin Piszcz <jpiszcz@...idpixels.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Yinghai Lu <yhlu.kernel@...il.com>
Subject: Re: [PATCH] trim memory not covered by WB MTRRs

Hi!

> On some machines, buggy BIOSes don't properly setup WB MTRRs to
> cover all available RAM, meaning the last few megs (or even gigs)
> of memory will be marked uncached.  Since Linux tends to allocate
> from high memory addresses first, this causes the machine to be
> unusably slow as soon as the kernel starts really using memory
> (i.e. right around init time).
> 
> This patch works around the problem by scanning the MTRRs at
> boot and figuring out whether the current end_pfn value (setup
> by early e820 code) goes beyond the highest WB MTRR range, and
> if so, trimming it to match.  A fairly obnoxious KERN_WARNING
> is printed too, letting the user know that not all of their
> memory is available due to a likely BIOS bug.
> 
> Something similar could be done on i386 if needed, but the boot
> ordering would be slightly different, since the MTRR code on i386
> depends on the boot_cpu_data structure being setup.
> 
> This patch fixes a bug in the last patch that caused the code to
> run on non-Intel machines (AMD machines apparently don't need it
> and it's untested on other non-Intel machines, so best keep it
> off).
> 
> akpm -- this one should replace all the mtrr patches currently
> in your tree.
> 
> Yinghai, maybe you can test this on one of your AMD machines to
> make sure I got the CPU code right?

> +	if ((highest_addr >> PAGE_SHIFT) != end_pfn) {
> +		printk(KERN_WARNING "***************\n");
> +		printk(KERN_WARNING "**** WARNING: likely BIOS bug\n");
> +		printk(KERN_WARNING "**** MTRRs don't cover all of "
> +		       "memory, trimmed %ld pages\n", end_pfn -
> +		       (highest_addr >> PAGE_SHIFT));
> +		printk(KERN_WARNING "***************\n");
> +		end_pfn = highest_addr >> PAGE_SHIFT;

I'd say using that many stars for KERN_WARNING printk is sign of
mental illness or something...
							Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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