[Bug 231760] FreeBSD 12.0-ALPHA7 Installations Halts at ACPI on 4 different AMD Ryzen Laptops (HP, DELL, Huawei)

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=231760

John Baldwin <jhb at FreeBSD.org> changed:

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                 CC|                            |markj at FreeBSD.org

--- Comment #16 from John Baldwin <jhb at FreeBSD.org> ---
I got access to two laptops at BSDCan tonight and figured out the bug.  I'll
clean up the patch I came up with and post it soonish.  However, the short
version is that the _PIC method in ACPI called a function that directly
accessed PCI config registers via the MCFG mapping to clear a bit when enabling
APIC mode.  However, since the BIOS didn't use PCI config space but directly
accessed MCFG directly via SystemMemory, AcpiOsMapMemory was used to map MCFG. 
This called pmap_mapbios() which remapped the MCFG window as Write-Back instead
of UC and the laptops hung right after this.  The fix is to add yet another
variant of pmap_mapdev() which leaves the existing PAT mode alone and doesn't
change it and change AcpiOsMapMemory to use that instead of pmap_mapbios.

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