Re: [PATCH v9 25/26] intel_idle/amx: Add SPR support with XTILEDATA capability

[Bae, Chang Seok]

On Jul 30, 2021, at 11:41, Hansen, Dave <dave.hansen@xxxxxxxxx> wrote:
>> +/**
>> + * idle_tile - Initialize TILE registers in INIT-state
>> + *
>> + * Leaving state in the dirty TILE registers may prevent the processor from
>> + * entering lower-power idle states. Use TILERELEASE to initialize the
>> + * state. Destroying fpregs state is safe after the fpstate update.
>> + */
>> +static inline void idle_tile(void)
>> +{
>> +	if (boot_cpu_has(X86_FEATURE_XGETBV1) && (xgetbv(1) & XFEATURE_MASK_XTILE)) {
>> +		tile_release();
>> +		fpregs_deactivate(&current->thread.fpu);
>> +	}
>> +}
> 
> This isn't obviously safe code.  There's a window in there when we have
> bogus, destroyed FPU register state but where we might be rescheduled.
> 
> I would assume that preempt is off *somewhere* in this, but it would be
> nice to make sure of that, or at least mention the requirement for it to
> be off before this code is safe.

I can see preempt_disable() in this path:

$kernel/sched/idle.c::play_idle_precise()
--> preempt_disable()
...
--> do_idle()
    --> cpuidle_idle_call()
        --> call_cpuidle()
            --> $drivers/cpuidle/cpuidle.c::cpuidle_enter()
                --> cpuidle_enter_state()
                    --> target_state->enter()
                        --> $drivers/idle/intel_idle.c::intel_idle_tile()
                            --> idle_tile()
...
--> preempt_enable()

> I'm also not sure TILERELEASE is *technically* what you want here.
> xgetbv(1) tells you whether a feature is being tracked by the processor
> as being in its init state.  tile_release() gets a feature into its init
> state, but does not guarantee that the processor will *track* it as
> being in the init state.
> 
> TILERELEASE is not documented to have an effect on XINUSE (init tracking).
> 
> XRSTOR, on the other hand, is at least documented to affect XINUSE.
> 
> It sounds like we either need a documentation update, or a clear
> explanation why TILERELEASE is being used over XRSTOR.

TILERELEASE impacts INIT tracking at least on the first AMX implementation. I
agree that the documentation needs some update.

Thanks,
Chang