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

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Message-Id: <200706211256.12551.jesse.barnes@intel.com>
Date:	Thu, 21 Jun 2007 12:56:12 -0700
From:	Jesse Barnes <jesse.barnes@...el.com>
To:	"Yinghai Lu" <yhlu.kernel@...il.com>
Cc:	"Andi Kleen" <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	"Justin Piszcz" <jpiszcz@...idpixels.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] trim memory not covered by WB MTRRs

On Thursday, June 21, 2007 12:40:58 Yinghai Lu wrote:
> On 6/7/07, Jesse Barnes <jesse.barnes@...el.com> wrote:
> > 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 incorporates the feedback from Eric and Andi:
> >   - use MAX_VAR_RANGES instead of NUM_VAR_RANGES
> >   - move array declaration to header file as an extern
> >   - add command line disable option "disable_mtrr_trim"
> >   - don't run the trim code if the MTRR default type is cacheable
> >   - don't run the trim code on non-Intel machines
> >
> > Justin, feel free to test again if you have time and add your
> > "Tested-by" signoff.
> >
> > Andi, as for large pages, do you think this is ok as is, or should
> > I trim a larger granularity?  If so, what granularity?
> >
> > Signed-off-by:  Jesse Barnes <jesse.barnes@...el.com>
> >
> > Thanks,
> > Jesse
>
> NAK.
>
> for AMD Rev F Opteron later CPU, BIOS will not set WB in MTRR for 4G
> above mem.
>
> This patch will get rid of those RAM.

Yeah, Eric already mentioned that.  I'll rework it to only run on Intel 
CPUs per Eric's last mail.

Thanks,
Jesse
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