[PATCH v2] x86/smpboot: Don't do mwait_play_dead() on AMD systems

[Yazen Ghannam]

From: Yazen Ghannam <yazen.ghannam@xxxxxxx>

Recent AMD systems support using MWAIT for C1 state. However, MWAIT will
not allow deeper cstates than C1 on current systems.

With play_dead() we expect the OS to use the deepest state available.
The deepest state available on AMD systems is reached through SystemIO
or HALT. If MWAIT is available, we use it instead of the other methods,
so we never reach the deepest state.

Don't try to use MWAIT to play_dead() on AMD systems. Instead, we'll use
CPUIDLE to enter the deepest state advertised by firmware. If CPUIDLE is
not available then we fallback to HALT.

Signed-off-by: Yazen Ghannam <yazen.ghannam@xxxxxxx>
---
Link:
https://lkml.kernel.org/r/20180402183424.48222-1-Yazen.Ghannam@xxxxxxx

v1->v2:
* Drop comment in code.

 arch/x86/kernel/smpboot.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index ff99e2b6fc54..12599e55e040 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1536,6 +1536,8 @@ static inline void mwait_play_dead(void)
 	void *mwait_ptr;
 	int i;
 
+	if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD)
+		return;
 	if (!this_cpu_has(X86_FEATURE_MWAIT))
 		return;
 	if (!this_cpu_has(X86_FEATURE_CLFLUSH))
-- 
2.14.1