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

Bae, Chang Seok
2021-08-03T21:32:37Z

On Jul 30, 2021, at 11:41, Hansen, Dave <dave.hansen@intel.com> 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