arm64/fpsimd: Permit kernel mode NEON with IRQs off

arm64/fpsimd: Permit kernel mode NEON with IRQs off

Currently, may_use_simd() will return false when called from a context
where IRQs are disabled. One notable case where this happens is when
calling the ResetSystem() EFI runtime service from the reboot/poweroff
code path. For this case alone, there is a substantial amount of FP/SIMD
support code to handle the corner case where a EFI runtime service is
invoked with IRQs disabled.

The only reason kernel mode SIMD is not allowed when IRQs are disabled
is that re-enabling softirqs in this case produces a noisy diagnostic
when lockdep is enabled. The warning is valid, in the sense that
delivering pending softirqs over the back of the call to
local_bh_enable() is problematic when IRQs are disabled.

While the API lacks a facility to simply mask and unmask softirqs
without triggering their delivery, disabling softirqs is not needed to
begin with when IRQs are disabled, given that softirqs are only every
taken asynchronously over the back of a hard IRQ.

So dis/enable softirq processing conditionally, based on whether IRQs
are enabled, and relax the check in may_use_simd().

Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>