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

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Date:   Tue, 3 Aug 2021 21:32:29 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     "Hansen, Dave" <dave.hansen@...el.com>
CC:     Borislav Petkov <bp@...e.de>, "Lutomirski, Andy" <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Macieira, Thiago" <thiago.macieira@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v9 25/26] intel_idle/amx: Add SPR support with XTILEDATA
 capability

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