Re: [PATCH v5 1/9] x86/split_lock: Rework the initialization flow of split lock detection

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

On 3/24/2020 6:29 PM, Thomas Gleixner wrote:
> Xiaoyao Li <xiaoyao.li@intel.com> writes:
>> On 3/24/2020 4:24 AM, Thomas Gleixner wrote:
>>> --- 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 = sld_off;
>>> +static DEFINE_PER_CPU(u64, msr_test_ctrl_cache);
>>
>> I used percpu cache in v3, but people prefer Tony's cache for reserved
>> bits[1].
>>
>> If you prefer percpu cache, I'll use it in next version.
> 
> I'm fine with the single variable.
> 
>>>    static void __init split_lock_setup(void)
>>>    {
>>>    	char arg[20];
>>>    	int i, ret;
>>>    
>>> +	if (!split_lock_verify_msr(true) || !split_lock_verify_msr(false)) {
>>> +		pr_info("MSR access failed: Disabled\n");
>>> +		return;
>>> +	}
>>> +
>>
>> I did similar thing like this in my v3, however Sean raised concern that
>> toggling MSR bit before parsing kernel param is bad behavior. [2]
> 
> That's trivial enough to fix.
> 
> Thanks,
> 
>          tglx
> 
> 8<---------------
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -44,7 +44,8 @@ enum split_lock_detect_state {
>    * split_lock_setup() will switch this to sld_warn on systems that support
>    * split lock detect, unless there is a command line override.
>    */
> -static enum split_lock_detect_state sld_state = sld_off;
> +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
> @@ -984,78 +985,85 @@ static inline bool match_option(const ch
>   	return len == arglen && !strncmp(arg, opt, len);
>   }
>   
> +static bool __init split_lock_verify_msr(bool on)
> +{
> +	u64 ctrl, tmp;
> +
> +	if (rdmsrl_safe(MSR_TEST_CTRL, &ctrl))
> +		return false;
> +	if (on)
> +		ctrl |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> +	else
> +		ctrl &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> +	if (wrmsrl_safe(MSR_TEST_CTRL, ctrl))
> +		return false;
> +	rdmsrl(MSR_TEST_CTRL, tmp);
> +	return ctrl == tmp;
> +}
> +
>   static void __init split_lock_setup(void)
>   {
> +	enum split_lock_detect_state state = sld_warn;
>   	char arg[20];
>   	int i, ret;
>   
> -	setup_force_cpu_cap(X86_FEATURE_SPLIT_LOCK_DETECT);
> -	sld_state = sld_warn;
> +	if (!split_lock_verify_msr(false)) {
> +		pr_info("MSR access failed: Disabled\n");
> +		return;
> +	}
>   
>   	ret = cmdline_find_option(boot_command_line, "split_lock_detect",
>   				  arg, sizeof(arg));
>   	if (ret >= 0) {
>   		for (i = 0; i < ARRAY_SIZE(sld_options); i++) {
>   			if (match_option(arg, ret, sld_options[i].option)) {
> -				sld_state = sld_options[i].state;
> +				state = sld_options[i].state;
>   				break;
>   			}
>   		}
>   	}
>   
> -	switch (sld_state) {
> +	switch (state) {
>   	case sld_off:
>   		pr_info("disabled\n");
> -		break;
> -
> +		return;
Here, when sld_off, it just returns without 
setup_force_cpu_cap(X86_FEATURE_SPLIT_LOCK_DETECT).

So for APs, it won't clear SLD bit in split_lock_init().

And I remember why I used sld_not_exist, not use
setup_force_cpu_cap(X86_FEATURE_SPLIT_LOCK_DETECT)

Yes, we can call setup_force_cpu_cap(X86_FEATURE_SPLIT_LOCK_DETECT)
for sld_off case. And in split_lock_init(), explicitly calling 
sld_update_msr(false) to turn off sld, and calling clear_cpu_cap(c, 
X86_FEATURE_SPLIT_LOCK_DETECT) to clear the cap. But due to 
setup_force_cpu_cap(), split_lock_detect will still occurs in 
/proc/cpuinfo.

>   	case sld_warn:
>   		pr_info("warning about user-space split_locks\n");
>   		break;
> -
>   	case sld_fatal:
>   		pr_info("sending SIGBUS on user-space split_locks\n");
>   		break;
>   	}
> +
> +	rdmsrl(MSR_TEST_CTRL, msr_test_ctrl_cache);
> +
> +	if (!split_lock_verify_msr(true)) {
> +		pr_info("MSR access failed: Disabled\n");
> +		return;
> +	}
> +
> +	sld_state = state;
> +	setup_force_cpu_cap(X86_FEATURE_SPLIT_LOCK_DETECT);
>   }
>   
>   /*
> - * Locking is not required at the moment because only bit 29 of this
> - * MSR is implemented and locking would not prevent that the operation
> - * of one thread is immediately undone by the sibling thread.
> - * Use the "safe" versions of rdmsr/wrmsr here because although code
> - * checks CPUID and MSR bits to make sure the TEST_CTRL MSR should
> - * exist, there may be glitches in virtualization that leave a guest
> - * with an incorrect view of real h/w capabilities.
> + * MSR_TEST_CTRL is per core, but we treat it like a per CPU MSR. Locking
> + * is not implemented as one thread could undo the setting of the other
> + * thread immediately after dropping the lock anyway.
>    */
> -static bool __sld_msr_set(bool on)
> +static void sld_update_msr(bool on)
>   {
> -	u64 test_ctrl_val;
> -
> -	if (rdmsrl_safe(MSR_TEST_CTRL, &test_ctrl_val))
> -		return false;
> +	u64 ctrl = 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;
> -
> -	return !wrmsrl_safe(MSR_TEST_CTRL, test_ctrl_val);
> +		ctrl |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> +	wrmsrl(MSR_TEST_CTRL, ctrl);
>   }
>   
>   static void split_lock_init(void)
>   {
> -	if (sld_state == sld_off)
> -		return;
> -
> -	if (__sld_msr_set(true))
> -		return;
> -
> -	/*
> -	 * If this is anything other than the boot-cpu, you've done
> -	 * funny things and you get to keep whatever pieces.
> -	 */
> -	pr_warn("MSR fail -- disabled\n");
> -	sld_state = sld_off;
> +	if (boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
> +		sld_update_msr(sld_state != sld_off);
>   }
>   
>   bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> @@ -1071,7 +1079,7 @@ bool handle_user_split_lock(struct pt_re
>   	 * progress and set TIF_SLD so the detection is re-enabled via
>   	 * switch_to_sld() when the task is scheduled out.
>   	 */
> -	__sld_msr_set(false);
> +	sld_update_msr(false);
>   	set_tsk_thread_flag(current, TIF_SLD);
>   	return true;
>   }
> @@ -1085,7 +1093,7 @@ bool handle_user_split_lock(struct pt_re
>    */
>   void switch_to_sld(unsigned long tifn)
>   {
> -	__sld_msr_set(!(tifn & _TIF_SLD));
> +	sld_update_msr(!(tifn & _TIF_SLD));
>   }
>   
>   #define SPLIT_LOCK_CPU(model) {X86_VENDOR_INTEL, 6, model, X86_FEATURE_ANY}
>