[PATCH v3 3/8] x86/split_lock: Cache the value of MSR_TEST_CTRL in percpu data

[Xiaoyao Li <xiaoyao.li@xxxxxxxxx>]

Cache the value of MSR_TEST_CTRL in percpu data msr_test_ctrl_cache,
which will be used by KVM module.

It also avoids an expensive RDMSR instruction if SLD needs to be context
switched.

Suggested-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
---
 arch/x86/include/asm/cpu.h  |  2 ++
 arch/x86/kernel/cpu/intel.c | 19 ++++++++++++-------
 2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
index ff567afa6ee1..2b20829db450 100644
--- a/arch/x86/include/asm/cpu.h
+++ b/arch/x86/include/asm/cpu.h
@@ -27,6 +27,8 @@ struct x86_cpu {
 };
 
 #ifdef CONFIG_HOTPLUG_CPU
+DECLARE_PER_CPU(u64, msr_test_ctrl_cache);
+
 extern int arch_register_cpu(int num);
 extern void arch_unregister_cpu(int);
 extern void start_cpu0(void);
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 49535ed81c22..ff27d026cb4a 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -46,6 +46,9 @@ enum split_lock_detect_state {
  */
 static enum split_lock_detect_state sld_state = sld_off;
 
+DEFINE_PER_CPU(u64, msr_test_ctrl_cache);
+EXPORT_PER_CPU_SYMBOL_GPL(msr_test_ctrl_cache);
+
 /*
  * Processors which have self-snooping capability can handle conflicting
  * memory type across CPUs by snooping its own cache. However, there exists
@@ -1043,20 +1046,22 @@ static void __init split_lock_setup(void)
  */
 static void __sld_msr_set(bool on)
 {
-	u64 test_ctrl_val;
-
-	rdmsrl(MSR_TEST_CTRL, test_ctrl_val);
-
 	if (on)
-		test_ctrl_val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
+		this_cpu_or(msr_test_ctrl_cache, MSR_TEST_CTRL_SPLIT_LOCK_DETECT);
 	else
-		test_ctrl_val &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
+		this_cpu_and(msr_test_ctrl_cache, ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT);
 
-	wrmsrl(MSR_TEST_CTRL, test_ctrl_val);
+	wrmsrl(MSR_TEST_CTRL, this_cpu_read(msr_test_ctrl_cache));
 }
 
 static void split_lock_init(void)
 {
+	u64 test_ctrl_val;
+
+	/* Cache MSR TEST_CTRL */
+	rdmsrl(MSR_TEST_CTRL, test_ctrl_val);
+	this_cpu_write(msr_test_ctrl_cache, test_ctrl_val);
+
 	if (sld_state == sld_off)
 		return;
 
-- 
2.23.0