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Subject[PATCH v4 02/10] x86/split_lock: Avoid runtime reads of the TEST_CTRL MSR
Date
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@intel.com>
Originally-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
---
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
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