Re: [PATCH v3 2/3] intel_idle: Sync up the SPEC_CTRL MSR value to x86_spec_ctrl_current

[Waiman Long <longman@redhat.com>]

On 6/22/23 01:46, Josh Poimboeuf wrote:
> On Wed, Jun 21, 2023 at 08:36:02PM -0400, Waiman Long wrote:
>> When intel_idle_ibrs() is called, it modifies the SPEC_CTRL MSR to
>> 0 in order disable IBRS. However, the new MSR value isn't reflected
>> in x86_spec_ctrl_current which is at odd with the other code that
>> keep track of its state in that percpu variable. Fix that by updating
>> x86_spec_ctrl_current percpu value to always match the content of the
>> SPEC_CTRL MSR.
> Is this fixing an actual bug or is there some other reason for doing
> this?
It is not a bug per se. It is mainly to make the per cpu variable more 
up to date.
>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   drivers/idle/intel_idle.c | 8 ++++++--
>>   1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
>> index aa2d19db2b1d..07fa23707b3c 100644
>> --- a/drivers/idle/intel_idle.c
>> +++ b/drivers/idle/intel_idle.c
>> @@ -181,13 +181,17 @@ static __cpuidle int intel_idle_ibrs(struct cpuidle_device *dev,
>>   	u64 spec_ctrl = spec_ctrl_current();
>>   	int ret;
>>   
>> -	if (smt_active)
>> +	if (smt_active) {
>> +		__this_cpu_write(x86_spec_ctrl_current, 0);
>>   		native_wrmsrl(MSR_IA32_SPEC_CTRL, 0);
>> +	}
>>   
>>   	ret = __intel_idle(dev, drv, index);
>>   
>> -	if (smt_active)
>> +	if (smt_active) {
>>   		native_wrmsrl(MSR_IA32_SPEC_CTRL, spec_ctrl);
>> +		__this_cpu_write(x86_spec_ctrl_current, spec_ctrl);
>> +	}
> More candidates for update_spec_ctrl()?

I don't think we can use update_spec_ctrl() here simply because of the 
noinstr requirement. See commit 9b461a6faae7 ("cpuidle, intel_idle: Fix 
CPUIDLE_FLAG_IBRS").

Cheers,
Longman