[PATCH v4 02/10] x86/split_lock: Avoid runtime reads of the TEST_CTRL MSR

[Xiaoyao Li <xiaoyao.li@xxxxxxxxx>]

In a context switch from a task that is detecting split locks
to one that is not (or vice versa) we need to update the TEST_CTRL
MSR. Currently this is done with the common sequence:
	read the MSR
	flip the bit
	write the MSR
in order to avoid changing the value of any reserved bits in the MSR.

Cache the value of the TEST_CTRL MSR when we read it during initialization
so we can avoid an expensive RDMSR instruction during context switch.

Suggested-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Originally-by: Tony Luck <tony.luck@xxxxxxxxx>
Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
---
 arch/x86/kernel/cpu/intel.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 064ba12defc8..4b3245035b5a 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -1020,6 +1020,14 @@ static void __init split_lock_setup(void)
 	}
 }
 
+/*
+ * Soft copy of MSR_TEST_CTRL initialized when we first read the
+ * MSR. Used at runtime to avoid using rdmsr again just to collect
+ * the reserved bits in the MSR. We assume reserved bits are the
+ * same on all CPUs.
+ */
+static u64 test_ctrl_val;
+
 /*
  * Locking is not required at the moment because only bit 29 of this
  * MSR is implemented and locking would not prevent that the operation
@@ -1027,16 +1035,14 @@ 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);
+	u64 val = test_ctrl_val;
 
 	if (on)
-		test_ctrl_val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
+		val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
 	else
-		test_ctrl_val &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
+		val &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
 
-	wrmsrl(MSR_TEST_CTRL, test_ctrl_val);
+	wrmsrl(MSR_TEST_CTRL, val);
 }
 
 /*
@@ -1048,11 +1054,13 @@ static void __sld_msr_set(bool on)
  */
 static void split_lock_init(struct cpuinfo_x86 *c)
 {
-	u64 test_ctrl_val;
+	u64 val;
 
-	if (rdmsrl_safe(MSR_TEST_CTRL, &test_ctrl_val))
+	if (rdmsrl_safe(MSR_TEST_CTRL, &val))
 		goto msr_broken;
 
+	test_ctrl_val = val;
+
 	switch (sld_state) {
 	case sld_off:
 		if (wrmsrl_safe(MSR_TEST_CTRL, test_ctrl_val & ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT))
-- 
2.20.1