[PATCH v3] arm64: fpsimd: improve stacking logic in non-interruptible context

[Dave.Martin@arm.com (Dave Martin)]

On Thu, Dec 08, 2016 at 04:49:39PM +0000, Ard Biesheuvel wrote:
> Currently, we allow kernel mode NEON in softirq or hardirq context by
> stacking and unstacking a slice of the NEON register file for each call
> to kernel_neon_begin() and kernel_neon_end(), respectively.
> 
> Given that
> a) a CPU typically spends most of its time in userland, during which time
>    no kernel mode NEON in process context is in progress,
> b) a CPU spends most of its time in the kernel doing other things than
>    kernel mode NEON when it gets interrupted to perform kernel mode NEON
>    in softirq context
> 
> the stacking and subsequent unstacking is only necessary if we are
> interrupting a thread while it is performing kernel mode NEON in process
> context, which means that in all other cases, we can simply preserve the
> userland FPSIMD state once, and only restore it upon return to userland,
> even if we are being invoked from softirq or hardirq context.
> 
> So instead of checking whether we are running in interrupt context, keep
> track of the level of nested kernel mode NEON calls in progress, and only
> perform the eager stack/unstack if the level exceeds 1.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
> v3:
> - avoid corruption by concurrent invocations of kernel_neon_begin()/_end()
> 
> v2:
> - BUG() on unexpected values of the nesting level
> - relax the BUG() on num_regs>32 to a WARN, given that nothing actually
>   breaks in that case
> 
>  arch/arm64/kernel/fpsimd.c | 47 ++++++++++++++------
>  1 file changed, 33 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index 394c61db5566..536ddab9316d 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -220,20 +220,35 @@ void fpsimd_flush_task_state(struct task_struct *t)
>  
>  #ifdef CONFIG_KERNEL_MODE_NEON
>  
> -static DEFINE_PER_CPU(struct fpsimd_partial_state, hardirq_fpsimdstate);
> -static DEFINE_PER_CPU(struct fpsimd_partial_state, softirq_fpsimdstate);
> +/*
> + * Although unlikely, it is possible for three kernel mode NEON contexts to
> + * be live at the same time: process context, softirq context and hardirq
> + * context. So while the userland context is stashed in the thread's fpsimd
> + * state structure, we need two additional levels of storage.
> + */
> +static DEFINE_PER_CPU(struct fpsimd_partial_state, nested_fpsimdstate[2]);
> +static DEFINE_PER_CPU(atomic_t, kernel_neon_nesting_level);

Come to think of it, should we use this_cpu_{read,add}() etc?

I'm not sure why we need the barrier semantics of atomic_t ops here.

[...]

Cheers
---Dave