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

[Luck, Tony]

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>
Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
---
 arch/x86/kernel/cpu/intel.c | 32 +++++++++++++++++++++++++-------
 1 file changed, 25 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 5d92e381fd91..78de69c5887a 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -1054,6 +1054,14 @@ 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
@@ -1063,19 +1071,29 @@ static void __init split_lock_setup(void)
  * exist, there may be glitches in virtualization that leave a guest
  * with an incorrect view of real h/w capabilities.
  */
-static bool __sld_msr_set(bool on)
+static bool __sld_msr_init(void)
 {
-	u64 test_ctrl_val;
+	u64 val;
 
-	if (rdmsrl_safe(MSR_TEST_CTRL, &test_ctrl_val))
+	if (rdmsrl_safe(MSR_TEST_CTRL, &val))
 		return false;
+	test_ctrl_val = val;
+
+	val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
+
+	return !wrmsrl_safe(MSR_TEST_CTRL, val);
+}
+
+static void __sld_msr_set(bool on)
+{
+	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;
 
-	return !wrmsrl_safe(MSR_TEST_CTRL, test_ctrl_val);
+	wrmsrl_safe(MSR_TEST_CTRL, val);
 }
 
 static void split_lock_init(void)
@@ -1083,7 +1101,7 @@ static void split_lock_init(void)
 	if (sld_state == sld_off)
 		return;
 
-	if (__sld_msr_set(true))
+	if (__sld_msr_init())
 		return;
 
 	/*
-- 
2.21.1