Fix from FreeBSD for atlhon problems with mtrr and XFree86. Ok deraadt@

FreeBSD commit messages say:

Some BIOSs are using MTRR values that are only documented under NDA
to control the mapping of things like the ACPI and APM into memory.

The problem is that starting X changes these values, so if something
was using the bits of BIOS mapped into memory (say ACPI or APM),
then next time they access this memory the machine would hang.

This patch refuse to change MTRR values it doesn't understand,
unless a new "force" option is given. This means X doesn't change
them by accident but someone can override that if they really want
to.

PR:             28418
Tested by:      Christopher Masto <chris at netmonger dot net>,
                David Bushong <david at bushong dot net>,
                Santos <casd at myrealbox dot com>


Make the MTRR code a bit more defensive - this should help people
trying to run X on some Athlon systems where the BIOS does odd things
(mines an ASUS A7A266, but it seems to also help on other systems).

Here's a description of the problem and my fix:

        The problem with the old MTRR code is that it only expects
        to find documented values in the bytes of MTRR registers.
        To convert the MTRR byte into a FreeBSD "Memory Range Type"
        (mrt) it uses the byte value and looks it up in an array.
        If the value is not in range then the mrt value ends up
        containing random junk.

        This isn't an immediate problem. The mrt value is only used
        later when rewriting the MTRR registers. When we finally
        go to write a value back again, the function i686_mtrrtype()
        searches for the junk value and returns -1 when it fails
        to find it. This is converted to a byte (0xff) and written
        back to the register, causing a GPF as 0xff is an illegal
        value for a MTRR byte.

        To work around this problem I've added a new mrt flag
        MDF_UNKNOWN.  We set this when we read a MTRR byte which
        we do not understand.  If we try to convert a MDF_UNKNOWN
        back into a MTRR value, then the new function, i686_mrt2mtrr,
        just returns the old value of the MTRR byte. This leaves
        the memory range type unchanged.

I have seen one side effect of the fix, which is that ACPI calls
after X has been run seem to hang my machine. As running X would
previously panic the machine, this is still an improvement ;-)

PR:             28418, 25958
Tested by:      jkh, Christopher Masto <chris at netmonger dot net>

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