Messages in this thread Patch in this message |  | | Date | Wed, 5 Feb 2020 16:49:44 -0800 | From | "Luck, Tony" <> | Subject | [PATCH] x86/split_lock: Avoid runtime reads of the TEST_CTRL MSR |
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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> Signed-off-by: Tony Luck <tony.luck@intel.com> --- 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
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