linux-kernel - [PATCH v1 2/4] Add enabling of the R3 MWAIT during boot for KNL

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Date:   Wed, 12 Oct 2016 12:13:08 +0200
From:   Grzegorz Andrejczuk <grzegorz.andrejczuk@...el.com>
To:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com, x86@...nel.org
Cc:     bp@...e.de, dave.hansen@...ux.intel.com,
        linux-kernel@...r.kernel.org, lukasz.daniluk@...el.com,
        james.h.cownie@...el.com,
        Grzegorz Andrejczuk <grzegorz.andrejczuk@...el.com>
Subject: [PATCH v1 2/4] Add enabling of the R3 MWAIT during boot for KNL

If processor is Intel Xeon Phi we enable user-level mwait feature.
Enabling this feature suppreses invalid-opcode error, when MONITOR/MWAIT
is called from ring 3.

Change-Id: I1c7defb99296b022790a068a6c725b3e860cd68c
Signed-off-by: Grzegorz Andrejczuk <grzegorz.andrejczuk@...el.com>
---
 arch/x86/kernel/cpu/intel.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index fcd484d..7f0f01a 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -61,6 +61,14 @@ void check_mpx_erratum(struct cpuinfo_x86 *c)
 	}
 }
 
+static int phir3mwait = 1;
+static int __init phir3mwait_disable(char *value)
+{
+	phir3mwait = 0;
+	return 1;
+}
+__setup("intel-phir3mwait=disable", phir3mwait_disable);
+
 static void early_init_intel(struct cpuinfo_x86 *c)
 {
 	u64 misc_enable;
@@ -211,6 +219,24 @@ static void early_init_intel(struct cpuinfo_x86 *c)
 	}
 
 	check_mpx_erratum(c);
+
+	/*
+	* Setting ring 3 MONITOR/MWAIT for all threads
+	* when CPU is Xeon Phi Family x200
+	* This can be disabled with phir3mwait=disable cmdline switch.
+	* We preserve the reserved values and set only 2nd bit.
+	* Ref:
+	* https://software.intel.com/en-us/blogs/2016/10/06/intel-xeon-phi-product-family-x200-knl-user-mode-ring-3-monitor-and-mwait
+	*/
+	if (c->x86 == 6 &&
+	    c->x86_model == INTEL_FAM6_XEON_PHI_KNL &&
+	    phir3mwait) {
+		u64 prev;
+
+		rdmsrl(MSR_PHI_MISC_THD_FEATURE_ENABLE, prev);
+		wrmsrl(MSR_PHI_MISC_THD_FEATURE_ENABLE,
+		       prev | MSR_PHI_MISC_THD_FEATURE_ENABLE_R3MWAIT);
+	}
 }
 
 #ifdef CONFIG_X86_32
-- 
2.5.1