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    SubjectRE: [PATCH] x86/microcode_intel_early.c: Get 32-bit physical address by __pa_nodebug()
    Date
    > From: yhlu.kernel@gmail.com [mailto:yhlu.kernel@gmail.com] On Behalf Of
    > Yinghai Lu
    > On Tue, Mar 19, 2013 at 8:04 AM, Fenghua Yu <fenghua.yu@intel.com>
    > wrote:
    > > From: Fenghua Yu <fenghua.yu@intel.com>
    > >
    > > In 32-bit, __pa_symbol() in CONFIG_DEBUG_VIRTUAL accesses kernel data
    > (e.g.
    > > max_low_pfn) that haven't been setup yet in such early boot phase. To
    > fix the
    > > issue, __pa_nodebug() replaces __pa_symbol() to get a global symbol's
    > physical
    > > address.
    >
    > That explanation is not precise.
    >
    > __phys_addr in 32bit when DEBUG_VIRTUAL is set, it does have check
    > /* max_low_pfn is set early, but not _that_ early */
    > if (max_low_pfn) {
    > ....
    > }
    > the reason should be:
    > in head_32.S, 32bit flat mode, you need to switch to phys addr to
    > access
    > global variable max_low_pfn.

    Max_low_pfn is not set up yet at this point, but __pa_symbol() reads it
    if CONFIG_DEBUG_VIRTUAL. There is no need to access max_low_pfn or any
    other data that is not setup yet if CONFIG_DEBUG_VIRTUAL.

    So I think the explanation is ok.

    Thanks.

    -Fenghua


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