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    SubjectRe: x86/mm: Limit 2/4M size calculation to x86_32
    On 25.07.2012 12:44, Avi Kivity wrote:
    > On 07/24/2012 06:52 PM, Cong Wang wrote:
    >
    >>> From 6b679d1af20656929c0e829f29eed60b0a86a74f Mon Sep 17 00:00:00 2001
    >>> From: Stefan Bader <stefan.bader@canonical.com>
    >>> Date: Fri, 13 Jul 2012 15:16:33 +0200
    >>> Subject: [PATCH] x86/mm: Limit 2/4M size calculation to x86_32
    >>>
    >>> commit 722bc6b (x86/mm: Fix the size calculation of mapping tables)
    >>> did modify the extra space calculation for mapping tables in order
    >>> to make up for the first 2/4M memory range using 4K pages.
    >>> However this setup is only used when compiling for 32bit. On 64bit
    >>> there is only the trailing area of 4K pages (which is already added).
    >>>
    >>> The code was already adapted once for things went wrong on a 8TB
    >>> machine (bd2753b x86/mm: Only add extra pages count for the first memory
    >>> range during pre-allocation early page table space), but it looks a bit
    >>> like it currently would overdo things for 64bit.
    >>> I only noticed while bisecting for the reason I could not make a crash
    >>> kernel boot (which ended up on this patch).
    >>>
    >>> Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
    >>> Cc: WANG Cong <xiyou.wangcong@gmail.com>
    >>> Cc: Yinghai Lu <yinghai@kernel.org>
    >>> Cc: Tejun Heo <tj@kernel.org>
    >>
    >> Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
    >>
    >> Sorry for that I was not aware of x86_64 is different with x86 in the
    >> first 2/4M.
    >
    > Why would there be a difference?
    >
    > Shouldn't the IO space at 0xa0000-0x100000 be mapped with uncacheable
    > attributes (or WC for VGA)? If it's done later, it can be done later
    > for both.
    >
    arch/x86/mm/init.c

    unsigned long __init_refok init_memory_mapping(...
    ...
    ifdef CONFIG_X86_32
    /*
    * Don't use a large page for the first 2/4MB of memory
    * because there are often fixed size MTRRs in there
    * and overlapping MTRRs into large pages can cause
    * slowdowns.
    */



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