x86/pat: Document the PAT initialization sequence
Update PAT documentation to describe how PAT is initialized under various configurations. Signed-off-by: Toshi Kani <toshi.kani@hpe.com> Reviewed-by: Thomas Gleixner <tglx@linutronix.de> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Borislav Petkov <bp@alien8.de> Cc: Borislav Petkov <bp@suse.de> Cc: Brian Gerst <brgerst@gmail.com> Cc: Denys Vlasenko <dvlasenk@redhat.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Juergen Gross <jgross@suse.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Luis R. Rodriguez <mcgrof@suse.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Toshi Kani <toshi.kani@hp.com> Cc: elliott@hpe.com Cc: konrad.wilk@oracle.com Cc: paul.gortmaker@windriver.com Cc: xen-devel@lists.xenproject.org Link: http://lkml.kernel.org/r/1458769323-24491-8-git-send-email-toshi.kani@hpe.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -196,3 +196,35 @@ Another, more verbose way of getting PAT related debug messages is with
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"debugpat" boot parameter. With this parameter, various debug messages are
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printed to dmesg log.
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PAT Initialization
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------------------
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The following table describes how PAT is initialized under various
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configurations. The PAT MSR must be updated by Linux in order to support WC
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and WT attributes. Otherwise, the PAT MSR has the value programmed in it
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by the firmware. Note, Xen enables WC attribute in the PAT MSR for guests.
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MTRR PAT Call Sequence PAT State PAT MSR
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=========================================================
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E E MTRR -> PAT init Enabled OS
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E D MTRR -> PAT init Disabled -
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D E MTRR -> PAT disable Disabled BIOS
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D D MTRR -> PAT disable Disabled -
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- np/E PAT -> PAT disable Disabled BIOS
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- np/D PAT -> PAT disable Disabled -
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E !P/E MTRR -> PAT init Disabled BIOS
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D !P/E MTRR -> PAT disable Disabled BIOS
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!M !P/E MTRR stub -> PAT disable Disabled BIOS
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Legend
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------------------------------------------------
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E Feature enabled in CPU
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D Feature disabled/unsupported in CPU
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np "nopat" boot option specified
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!P CONFIG_X86_PAT option unset
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!M CONFIG_MTRR option unset
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Enabled PAT state set to enabled
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Disabled PAT state set to disabled
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OS PAT initializes PAT MSR with OS setting
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BIOS PAT keeps PAT MSR with BIOS setting
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