[tip: x86/urgent] x86/asm: Fix MWAITX C-state hint value

[tip-bot2 for Janakarajan Natarajan]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     9b69cab42e5d14b8f0467566e3d97e682365db2d
Gitweb:        https://git.kernel.org/tip/9b69cab42e5d14b8f0467566e3d97e682365db2d
Author:        Janakarajan Natarajan <Janakarajan.Natarajan@xxxxxxx>
AuthorDate:    Mon, 07 Oct 2019 19:00:22 
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Tue, 08 Oct 2019 09:48:09 +02:00

x86/asm: Fix MWAITX C-state hint value

As per "AMD64 Architecture Programmer's Manual Volume 3: General-Purpose
and System Instructions", MWAITX EAX[7:4]+1 specifies the optional hint
of the optimized C-state. For C0 state, EAX[7:4] should be set to 0xf.

Currently, a value of 0xf is set for EAX[3:0] instead of EAX[7:4]. Fix
this by changing MWAITX_DISABLE_CSTATES from 0xf to 0xf0.

This hasn't had any implications so far because setting reserved bits in
EAX is simply ignored by the CPU.

 [ bp: Fixup comment in delay_mwaitx() and massage. ]

Signed-off-by: Janakarajan Natarajan <Janakarajan.Natarajan@xxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: "x86@xxxxxxxxxx" <x86@xxxxxxxxxx>
Cc: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20191007190011.4859-1-Janakarajan.Natarajan@xxxxxxx
---
 arch/x86/include/asm/mwait.h | 2 +-
 arch/x86/lib/delay.c         | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
index e28f8b7..9d5252c 100644
--- a/arch/x86/include/asm/mwait.h
+++ b/arch/x86/include/asm/mwait.h
@@ -21,7 +21,7 @@
 #define MWAIT_ECX_INTERRUPT_BREAK	0x1
 #define MWAITX_ECX_TIMER_ENABLE		BIT(1)
 #define MWAITX_MAX_LOOPS		((u32)-1)
-#define MWAITX_DISABLE_CSTATES		0xf
+#define MWAITX_DISABLE_CSTATES		0xf0
 
 static inline void __monitor(const void *eax, unsigned long ecx,
 			     unsigned long edx)
diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
index b7375dc..c126571 100644
--- a/arch/x86/lib/delay.c
+++ b/arch/x86/lib/delay.c
@@ -113,8 +113,8 @@ static void delay_mwaitx(unsigned long __loops)
 		__monitorx(raw_cpu_ptr(&cpu_tss_rw), 0, 0);
 
 		/*
-		 * AMD, like Intel, supports the EAX hint and EAX=0xf
-		 * means, do not enter any deep C-state and we use it
+		 * AMD, like Intel's MWAIT version, supports the EAX hint and
+		 * EAX=0xf0 means, do not enter any deep C-state and we use it
 		 * here in delay() to minimize wakeup latency.
 		 */
 		__mwaitx(MWAITX_DISABLE_CSTATES, delay, MWAITX_ECX_TIMER_ENABLE);