@@ -196,3 +196,35 @@ Another, more verbose way of getting PAT related debug messages is with
"debugpat" boot parameter. With this parameter, various debug messages are
printed to dmesg log.
+PAT Initialization
+------------------
+
+The following table describes how PAT is initialized under various
+configurations. PAT MSR must be updated by Linux in order to support WC
+and WT attributes. Otherwise, the PAT MSR has the value programmed in it
+by the firmware. Note, Xen enables WC attribute in the PAT MSR for guests.
+
+ MTRR PAT Call Sequence PAT State PAT MSR
+ =========================================================
+ E E MTRR -> PAT init Enabled OS
+ E D MTRR -> PAT init Disabled -
+ D E MTRR -> PAT disable Disabled BIOS
+ D D MTRR -> PAT disable Disabled -
+ - np/E PAT -> PAT disable Disabled BIOS
+ - np/D PAT -> PAT disable Disabled -
+ E !P/E MTRR -> PAT init Disabled BIOS
+ D !P/E MTRR -> PAT disable Disabled BIOS
+ !M !P/E MTRR stub -> PAT disable Disabled BIOS
+
+ Legend
+ ------------------------------------------------
+ E Feature enabled in CPU
+ D Feature disabled/unsupported in CPU
+ np "nopat" boot option specified
+ !P CONFIG_X86_PAT option unset
+ !M CONFIG_MTRR option unset
+ Enabled PAT state set to enable
+ Disabled PAT state set to disable
+ OS PAT initializes PAT MSR with OS setting
+ BIOS PAT keeps PAT MSR with BIOS setting
+
Update PAT documentation to describe how PAT is initialized under various configurations. Signed-off-by: Toshi Kani <toshi.kani@hpe.com> Cc: Borislav Petkov <bp@suse.de> Cc: Luis R. Rodriguez <mcgrof@suse.com> Cc: Juergen Gross <jgross@suse.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Thomas Gleixner <tglx@linutronix.de> --- Documentation/x86/pat.txt | 32 ++++++++++++++++++++++++++++++++ 1 file changed, 32 insertions(+)