may_use_simd on aarch64, chacha20

[Jason A. Donenfeld]

Hi folks,

I noticed that the ARM implementation [1] of chacha20 makes a check to
may_use_simd(), but the ARM64 implementation [2] does not. Question 1:
is this a bug, in which case I'll submit a patch shortly, or is this
intentional? In case of the latter, could somebody explain the
reasoning? On a similar note, the only ARM64 glue code that uses
may_use_simd() is sha256; everything else does not. Shall I submit a
substantial patch series to fix this up everywhere?

Secondly, I noticed that may_use_simd() is essentially aliased to
!in_interrupt(), since it uses the asm-generic variety. Question 2:
Isn't this overkill? Couldn't we make an arm/arm64 variant of this
that only checks in_irq()?

Lastly, APIs like pcrypts and padata execute with bottom halves
disabled, even though their actual execution environment is process
context, via a workqueue. Thus, here, in_interrupt() will always be
true, even though this is likely a place where we want to use simd.
Question 3: is there something better that could be done?

Thanks,
Jason

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm/crypto/chacha20-neon-glue.c#n67
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/crypto/chacha20-neon-glue.c#n66