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    SubjectRe: [PATCH] trim memory not covered by WB MTRRs


    On Thu, 5 Jul 2007, Pavel Machek wrote:

    > 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
    >

    Will this patch make it into 2.6.23?

    Been patching manually for a while with each -rc for 2.6.22..

    Justin.
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