[PATCH v7 2/2] x86/split_lock: Avoid runtime reads of the TEST_CTRL MSR

[Xiaoyao Li]

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 unused and reserved bits of TEST_CTRL MSR with SPLIT_LOCK_DETECT
bit cleared 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 | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index deb5c42c2089..1f414578899c 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -45,6 +45,7 @@ enum split_lock_detect_state {
  * split lock detect, unless there is a command line override.
  */
 static enum split_lock_detect_state sld_state __ro_after_init = sld_off;
+static u64 msr_test_ctrl_cache __ro_after_init;
 
 /*
  * Processors which have self-snooping capability can handle conflicting
@@ -1037,6 +1038,8 @@ static void __init split_lock_setup(void)
 		break;
 	}
 
+	rdmsrl(MSR_TEST_CTRL, msr_test_ctrl_cache);
+
 	if (!split_lock_verify_msr(true)) {
 		pr_info("MSR access failed: Disabled\n");
 		return;
@@ -1053,14 +1056,10 @@ static void __init split_lock_setup(void)
  */
 static void sld_update_msr(bool on)
 {
-	u64 test_ctrl_val;
-
-	rdmsrl(MSR_TEST_CTRL, test_ctrl_val);
+	u64 test_ctrl_val = msr_test_ctrl_cache;
 
 	if (on)
 		test_ctrl_val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
-	else
-		test_ctrl_val &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
 
 	wrmsrl(MSR_TEST_CTRL, test_ctrl_val);
 }
-- 
2.20.1