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

On 13 December 2016 at 13:49, Dave Martin [off-list ref] wrote:

On Tue, Dec 13, 2016 at 12:35:29PM +0000, Ard Biesheuvel wrote:

quoted

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.

Hang on ... we could be in the middle of fpsimd_save_state() for other
reasons, say on the context switch path.  Wouldn't we need to take the
lock for those too?

Yes. Saving the state should never be interrupted, and now that I
think of it, this is an issue for your original patch as well: even if
you always preserve the state first in kernel_neon_begin(), any
occurrence of fpsimd_save_state() could be interrupted by a
kernel_neon_begin()/_end() pair, after which the SVE state is nuked
anyway (regardless of whether a patch like this one is applied) So I
think we do need the lock in all cases where the FP/SIMD is copied to
memory and the flag set.

Also, a spinlock can't protect a critical section from code running on
the same CPU... wouldn't it just deadlock in that case?

That is why I use spin_trylock(). But I realise now that the patch is
incorrect: the nesting level needs to be incremented first, or the
interrupter will still clobber the context while it is being
preserved.

I still tend to prefer v4 -- there we do a redundant double-save only if
one kernel_neon_begin() interrupts the actual task context save.

I think we're not at the bottom of this rabbit hole yet:-)