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 4:24 AM, Thomas Gleixner wrote:
> Thomas Gleixner <tglx@linutronix.de> writes:
>> Xiaoyao Li <xiaoyao.li@intel.com> writes:
>>
>>> Current initialization flow of split lock detection has following issues:
>>> 1. It assumes the initial value of MSR_TEST_CTRL.SPLIT_LOCK_DETECT to be
>>>     zero. However, it's possible that BIOS/firmware has set it.
>>
>> Ok.
>>
>>> 2. X86_FEATURE_SPLIT_LOCK_DETECT flag is unconditionally set even if
>>>     there is a virtualization flaw that FMS indicates the existence while
>>>     it's actually not supported.
>>>
>>> 3. Because of #2, KVM cannot rely on X86_FEATURE_SPLIT_LOCK_DETECT flag
>>>     to check verify if feature does exist, so cannot expose it to
>>>     guest.
>>
>> Sorry this does not make anny sense. KVM is the hypervisor, so it better
>> can rely on the detect flag. Unless you talk about nested virt and a
>> broken L1 hypervisor.
>>
>>> To solve these issues, introducing a new sld_state, "sld_not_exist",
>>> as
>>
>> The usual naming convention is sld_not_supported.
> 
> But this extra state is not needed at all, it already exists:
> 
>      X86_FEATURE_SPLIT_LOCK_DETECT
> 
> You just need to make split_lock_setup() a bit smarter. Soemthing like
> the below. It just wants to be split into separate patches.
> 
> Thanks,
> 
>          tglx
> ---
> --- 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.

[1]: https://lore.kernel.org/lkml/20200303192242.GU1439@linux.intel.com/

>   /*
>    * Processors which have self-snooping capability can handle conflicting
> @@ -984,11 +985,32 @@ 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)
>   {
>   	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]

[2]: https://lore.kernel.org/kvm/20200305162311.GG11500@linux.intel.com/