[PATCH v6] arm64: fpsimd: improve stacking logic in non-interruptible context

On Tue, Dec 13, 2016 at 12:35:29PM +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 <redacted>
---
v6:
- use a spinlock instead of disabling interrupts

My concern with the spinlock or disabling interrupts is the latency if
we ever get some insanely long SVE vectors.

-- 
Catalin