[PATCH v3 2/7] x86/mm/pat: Add pat_disable() interface

[Toshi Kani]

In preparation for fixing a regression caused by 'commit 9cd25aac1f44
("x86/mm/pat: Emulate PAT when it is disabled")', PAT needs to
provide an interface that prevents the OS from initializing the
PAT MSR.

PAT MSR initialization must be done on all CPUs using the specific
sequence of operations defined in Intel SDM.  This requires MTRRs
to be enabled since pat_init() is called as part of MTRR init
from mtrr_rendezvous_handler().

Make pat_disable() as the interface that prevents the OS from
initializing the PAT MSR.  MTRR will call this interface when it
cannot provide the SDM-defined sequence to initialize PAT.

This also assures pat_disable() called from pat_bsp_init() to
set PAT table properly when CPU does not support PAT.

Signed-off-by: Toshi Kani <toshi.kani@xxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Luis R. Rodriguez <mcgrof@xxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Robert Elliott <elliott@xxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 arch/x86/include/asm/pat.h |    1 +
 arch/x86/mm/pat.c          |   13 ++++++++++++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/pat.h b/arch/x86/include/asm/pat.h
index 97ea55b..0ad356c 100644
--- a/arch/x86/include/asm/pat.h
+++ b/arch/x86/include/asm/pat.h
@@ -5,6 +5,7 @@
 #include <asm/pgtable_types.h>
 
 bool pat_enabled(void);
+void pat_disable(const char *reason);
 extern void pat_init(void);
 void __init_cache_modes(u64);
 
diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index 1da55a5..3c08a27 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -40,11 +40,22 @@
 static bool boot_cpu_done;
 
 static int __read_mostly __pat_enabled = IS_ENABLED(CONFIG_X86_PAT);
+static void init_cache_modes(void);
 
-static inline void pat_disable(const char *reason)
+void pat_disable(const char *reason)
 {
+	if (!__pat_enabled)
+		return;
+
+	if (boot_cpu_done) {
+		WARN_ONCE(1, "x86/PAT: PAT cannot be disabled after initialization\n");
+		return;
+	}
+
 	__pat_enabled = 0;
 	pr_info("x86/PAT: %s\n", reason);
+
+	init_cache_modes();
 }
 
 static int __init nopat(char *str)