linux-kernel - [PATCH 2/2] x86: Enable monitor/mwait on Intel if BIOS hasn't already

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Message-Id: <5bec80947da3fb72f2f18fbe6d924a69f83fbed9.1312386029.git.luto@mit.edu>
Date:	Wed,  3 Aug 2011 11:47:17 -0400
From:	Andy Lutomirski <luto@....EDU>
To:	x86@...nel.org, linux-kernel@...r.kernel.org
Cc:	Fenghua Yu <fenghua.yu@...el.com>, Andy Lutomirski <luto@....edu>
Subject: [PATCH 2/2] x86: Enable monitor/mwait on Intel if BIOS hasn't already

I have a Sandy Bridge machine with an Intel BIOS that enables
monitor/mwait on all but the boot CPU.  With this patch, intel_idle
works.

Signed-off-by: Andy Lutomirski <luto@....edu>
---
 arch/x86/kernel/cpu/intel.c |   20 ++++++++++++++++++++
 1 files changed, 20 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 6b20fef..a3339a1 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -487,6 +487,26 @@ static void __cpuinit init_intel(struct cpuinfo_x86 *c)
 			wrmsrl(MSR_IA32_ENERGY_PERF_BIAS, epb);
 		}
 	}
+
+	/* Enable monitor/mwait if BIOS didn't do it for us. */
+	if (!cpu_has(c, X86_FEATURE_MWAIT) && cpu_has(c, X86_FEATURE_XMM3)
+	    && c->x86 >= 6 && !(c->x86 == 6 && c->x86_model < 0x1c)
+	    && !(c->x86 == 0xf && c->x86_model < 3)) {
+		u64 misc_enable;
+		rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
+		misc_enable |= MSR_IA32_MISC_ENABLE_MWAIT;
+
+		/*
+		 * Some non-SSE3 cpus will #GP.  We check for that,
+		 * but it can't hurt to be safe.
+		 */
+		wrmsr_safe(MSR_IA32_MISC_ENABLE, (u32)misc_enable,
+			   (u32)(misc_enable >> 32));
+
+		/* Re-read monitor capability. */
+		if (cpuid_ecx(1) & 0x8)
+			set_cpu_cap(c, X86_FEATURE_MWAIT);
+	}
 }
 
 #ifdef CONFIG_X86_32
-- 
1.7.6

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