[RFC PATCH] arm64: neon: Remove support for nested or hardirq kernel-mode NEON

[Mark Rutland]

Hi,

On Fri, May 19, 2017 at 12:26:39PM +0100, Dave Martin wrote:
> +static bool __maybe_unused kernel_neon_allowed(void)
> +{
> +	/*
> +	 * The per_cpu_ptr() is racy if called with preemption enabled.
> +	 * This is not a bug:  per_cpu(kernel_neon_busy) is only set
> +	 * when preemption is disabled, so we cannot migrate to another
> +	 * CPU while it is set, nor can we migrate to a CPU where it is set.
> +	 * So, if we find it clear on some CPU then we're guaranteed to find it
> +	 * clear on any CPU we could migrate to.
> +	 *
> +	 * If we are in between kernel_neon_begin()...kernel_neon_end(), the
> +	 * flag will be set, but preemption is also disabled, so we can't
> +	 * migrate to another CPU and spuriously see it become false.
> +	 */
> +	return !(in_irq() || in_nmi()) &&
> +		!local_read(this_cpu_ptr(&kernel_neon_busy));
> +}

I think it would be better to use the this_cpu ops for this, rather than
manually messing with the pointer.

Here, we can use raw_cpu_read(kernel_neon_busy), given the comment
above.

[...]

> +DEFINE_PER_CPU(local_t, kernel_neon_busy);

This can become a basic type like int or bool.

[...]

> +void kernel_neon_begin(void)
>  {
>  	if (WARN_ON(!system_supports_fpsimd()))
>  		return;
> -	if (in_interrupt()) {
> -		struct fpsimd_partial_state *s = this_cpu_ptr(
> -			in_irq() ? &hardirq_fpsimdstate : &softirq_fpsimdstate);
>  
> -		BUG_ON(num_regs > 32);
> -		fpsimd_save_partial_state(s, roundup(num_regs, 2));
> -	} else {
> -		/*
> -		 * Save the userland FPSIMD state if we have one and if we
> -		 * haven't done so already. Clear fpsimd_last_state to indicate
> -		 * that there is no longer userland FPSIMD state in the
> -		 * registers.
> -		 */
> -		preempt_disable();
> -		if (current->mm &&
> -		    !test_and_set_thread_flag(TIF_FOREIGN_FPSTATE))
> -			fpsimd_save_state(&current->thread.fpsimd_state);
> +	BUG_ON(!kernel_neon_allowed());
> +
> +	preempt_disable();
> +	local_set(this_cpu_ptr(&kernel_neon_busy), 1);

As preemption is disabled:

	__this_cpu_write(kernel_neon_busy, 1);

[...]

>  void kernel_neon_end(void)
>  {
> +	long busy;
> +
>  	if (!system_supports_fpsimd())
>  		return;
> -	if (in_interrupt()) {
> -		struct fpsimd_partial_state *s = this_cpu_ptr(
> -			in_irq() ? &hardirq_fpsimdstate : &softirq_fpsimdstate);
> -		fpsimd_load_partial_state(s);
> -	} else {
> -		preempt_enable();
> -	}
> +
> +	busy = local_xchg(this_cpu_ptr(&kernel_neon_busy), 0);

Likewise:

	busy = __this_cpu_xchg(kernel_neon_busy, 0);

Thanks,
Mark.