Re: [PATCH v3 3/8] x86/split_lock: Cache the value of MSR_TEST_CTRL in percpu data

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 3/4/2020 3:18 AM, Sean Christopherson wrote:
> On Thu, Feb 06, 2020 at 03:04:07PM +0800, Xiaoyao Li wrote:
>> Cache the value of MSR_TEST_CTRL in percpu data msr_test_ctrl_cache,
>> which will be used by KVM module.
>>
>> It also avoids an expensive RDMSR instruction if SLD needs to be context
>> switched.
>>
>> Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
>> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
>> ---
>>   arch/x86/include/asm/cpu.h  |  2 ++
>>   arch/x86/kernel/cpu/intel.c | 19 ++++++++++++-------
>>   2 files changed, 14 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
>> index ff567afa6ee1..2b20829db450 100644
>> --- a/arch/x86/include/asm/cpu.h
>> +++ b/arch/x86/include/asm/cpu.h
>> @@ -27,6 +27,8 @@ struct x86_cpu {
>>   };
>>   
>>   #ifdef CONFIG_HOTPLUG_CPU
>> +DECLARE_PER_CPU(u64, msr_test_ctrl_cache);
>> +
>>   extern int arch_register_cpu(int num);
>>   extern void arch_unregister_cpu(int);
>>   extern void start_cpu0(void);
>> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
>> index 49535ed81c22..ff27d026cb4a 100644
>> --- a/arch/x86/kernel/cpu/intel.c
>> +++ b/arch/x86/kernel/cpu/intel.c
>> @@ -46,6 +46,9 @@ enum split_lock_detect_state {
>>    */
>>   static enum split_lock_detect_state sld_state = sld_off;
>>   
>> +DEFINE_PER_CPU(u64, msr_test_ctrl_cache);
>> +EXPORT_PER_CPU_SYMBOL_GPL(msr_test_ctrl_cache);
>> +
>>   /*
>>    * Processors which have self-snooping capability can handle conflicting
>>    * memory type across CPUs by snooping its own cache. However, there exists
>> @@ -1043,20 +1046,22 @@ 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);
>> -
>>   	if (on)
>> -		test_ctrl_val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
>> +		this_cpu_or(msr_test_ctrl_cache, MSR_TEST_CTRL_SPLIT_LOCK_DETECT);
>>   	else
>> -		test_ctrl_val &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
>> +		this_cpu_and(msr_test_ctrl_cache, ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT);
> 
> Updating the cache is at best unnecessary, and at worst dangerous, e.g. it
> incorrectly implies that the cached value of SPLIT_LOCK_DETECT is reliable.
> 
> Tony's patch[*] is more what I had in mind, the only question is whether the
> kernel should be paranoid about other bits in MSR_TEST_CTL.

OK, I'll use Tony's patch.

> [*] 20200206004944.GA11455@agluck-desk2.amr.corp.intel.com
> 
>> -	wrmsrl(MSR_TEST_CTRL, test_ctrl_val);
>> +	wrmsrl(MSR_TEST_CTRL, this_cpu_read(msr_test_ctrl_cache));
>>   }
>>   
>>   static void split_lock_init(void)
>>   {
>> +	u64 test_ctrl_val;
>> +
>> +	/* Cache MSR TEST_CTRL */
>> +	rdmsrl(MSR_TEST_CTRL, test_ctrl_val);
>> +	this_cpu_write(msr_test_ctrl_cache, test_ctrl_val);
>> +
>>   	if (sld_state == sld_off)
>>   		return;
>>   
>> -- 
>> 2.23.0
>>