[PATCH v5 1/3] x86/smp: Allow calling mwait_play_dead with an arbitrary hint

[Patryk Wlazlyn <patryk.wlazlyn@linux.intel.com>]

The MWAIT instruction needs different hints on different CPUs to reach
specific idle states. The current hint calculation* in mwait_play_dead()
code works in practice on current Intel hardware, but it fails on a
recent one, Intel's Sierra Forest and possibly some future ones.  Those
newer CPUs' power efficiency suffers when the CPU is put offline.

* The current algorithm for looking up the mwait hint for the deepest
cstate, in mwait_play_dead() code works by inspecting CPUID leaf 0x5 and
calculates the mwait hint based on the number of reported substates.
This approach depends on the hints associated with them to be continuous
in the range [0, NUM_SUBSTATES-1]. This continuity is not documented and
is not met on the recent Intel platforms.

For example, Intel's Sierra Forest report two cstates with two substates
each in cpuid leaf 0x5:

  Name*   target cstate    target subcstate (mwait hint)
  ===========================================================
  C1      0x00             0x00
  C1E     0x00             0x01

  --      0x10             ----

  C6S     0x20             0x22
  C6P     0x20             0x23

  --      0x30             ----

  /* No more (sub)states all the way down to the end. */
  ===========================================================

   * Names of the cstates are not included in the CPUID leaf 0x5, they are
     taken from the product specific documentation.

Notice that hints 0x20 and 0x21 are skipped entirely for the target
cstate 0x20 (C6), being a cause of the problem for the current cpuid
leaf 0x5 algorithm.

Allow cpuidle code to call mwait play dead loop with a known hint for
the deepest idle state on a given platform, skipping the cpuid based
calculation.

Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@linux.intel.com>
---
 arch/x86/include/asm/smp.h |  3 +++
 arch/x86/kernel/smpboot.c  | 46 +++++---------------------------------
 2 files changed, 8 insertions(+), 41 deletions(-)

diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
index ca073f40698f..633b4a4aec6b 100644
--- a/arch/x86/include/asm/smp.h
+++ b/arch/x86/include/asm/smp.h
@@ -114,6 +114,7 @@ void wbinvd_on_cpu(int cpu);
 int wbinvd_on_all_cpus(void);
 
 void smp_kick_mwait_play_dead(void);
+void mwait_play_dead(unsigned long hint);
 
 void native_smp_send_reschedule(int cpu);
 void native_send_call_func_ipi(const struct cpumask *mask);
@@ -164,6 +165,8 @@ static inline struct cpumask *cpu_llc_shared_mask(int cpu)
 {
 	return (struct cpumask *)cpumask_of(0);
 }
+
+static inline void mwait_play_dead(unsigned long eax_hint) { }
 #endif /* CONFIG_SMP */
 
 #ifdef CONFIG_DEBUG_NMI_SELFTEST
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index b5a8f0891135..5dc143e1d6af 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1276,45 +1276,9 @@ void play_dead_common(void)
  * We need to flush the caches before going to sleep, lest we have
  * dirty data in our caches when we come back up.
  */
-static inline void mwait_play_dead(void)
+void __noreturn mwait_play_dead(unsigned long eax_hint)
 {
 	struct mwait_cpu_dead *md = this_cpu_ptr(&mwait_cpu_dead);
-	unsigned int eax, ebx, ecx, edx;
-	unsigned int highest_cstate = 0;
-	unsigned int highest_subcstate = 0;
-	int i;
-
-	if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
-	    boot_cpu_data.x86_vendor == X86_VENDOR_HYGON)
-		return;
-	if (!this_cpu_has(X86_FEATURE_MWAIT))
-		return;
-	if (!this_cpu_has(X86_FEATURE_CLFLUSH))
-		return;
-	if (__this_cpu_read(cpu_info.cpuid_level) < CPUID_MWAIT_LEAF)
-		return;
-
-	eax = CPUID_MWAIT_LEAF;
-	ecx = 0;
-	native_cpuid(&eax, &ebx, &ecx, &edx);
-
-	/*
-	 * eax will be 0 if EDX enumeration is not valid.
-	 * Initialized below to cstate, sub_cstate value when EDX is valid.
-	 */
-	if (!(ecx & CPUID5_ECX_EXTENSIONS_SUPPORTED)) {
-		eax = 0;
-	} else {
-		edx >>= MWAIT_SUBSTATE_SIZE;
-		for (i = 0; i < 7 && edx; i++, edx >>= MWAIT_SUBSTATE_SIZE) {
-			if (edx & MWAIT_SUBSTATE_MASK) {
-				highest_cstate = i;
-				highest_subcstate = edx & MWAIT_SUBSTATE_MASK;
-			}
-		}
-		eax = (highest_cstate << MWAIT_SUBSTATE_SIZE) |
-			(highest_subcstate - 1);
-	}
 
 	/* Set up state for the kexec() hack below */
 	md->status = CPUDEAD_MWAIT_WAIT;
@@ -1335,7 +1299,7 @@ static inline void mwait_play_dead(void)
 		mb();
 		__monitor(md, 0, 0);
 		mb();
-		__mwait(eax, 0);
+		__mwait(eax_hint, 0);
 
 		if (READ_ONCE(md->control) == CPUDEAD_MWAIT_KEXEC_HLT) {
 			/*
@@ -1407,9 +1371,9 @@ void native_play_dead(void)
 	play_dead_common();
 	tboot_shutdown(TB_SHUTDOWN_WFS);
 
-	mwait_play_dead();
-	if (cpuidle_play_dead())
-		hlt_play_dead();
+	/* Below returns only on error. */
+	cpuidle_play_dead();
+	hlt_play_dead();
 }
 
 #else /* ... !CONFIG_HOTPLUG_CPU */
-- 
2.47.1