arm64/fpsimd: Clarify sve_sync_*() functions

The sve_sync_{to,from}_fpsimd*() functions are intended to
extract/insert the currently effective FPSIMD state of a task regardless
of whether the task's state is saved in FPSIMD format or SVE format.
Historically they were only used by ptrace, but sve_sync_to_fpsimd() is
now used more widely, and sve_sync_from_fpsimd_zeropad() may be used
more widely in future.

When FPSIMD/SVE state tracking was changed across commits:

  baa85152 ("arm64/fpsimd: Track the saved FPSIMD state type separately to TIF_SVE")
  a0136be4 (arm64/fpsimd: Load FP state based on recorded data type")
  bbc6172e ("arm64/fpsimd: SME no longer requires SVE register state")
  8c845e27 ("arm64/sve: Leave SVE enabled on syscall if we don't context switch")

... sve_sync_to_fpsimd() was updated to consider task->thread.fp_type
rather than the task's TIF_SVE and PSTATE.SM, but (apparently due to an
oversight) sve_sync_from_fpsimd_zeropad() was left as-is, leaving the
two inconsistent.

Due to this, sve_sync_from_fpsimd_zeropad() may copy state from
task->thread.uw.fpsimd_state into task->thread.sve_state when
task->thread.fp_type == FP_STATE_FPSIMD. This is redundant (but benign)
as task->thread.uw.fpsimd_state is the effective state that will be
restored, and task->thread.sve_state will not be consumed. For
consistency, and to avoid the redundant work, it better for
sve_sync_from_fpsimd_zeropad() to consider task->thread.fp_type alone,
matching sve_sync_to_fpsimd().

The naming of both functions is somehat unfortunate, as it is unclear
when and why they copy state. It would be better to describe them in
terms of the effective state.

Considering all of the above, clean this up:

* Adjust sve_sync_from_fpsimd_zeropad() to consider
  task->thread.fp_type.

* Update comments to clarify the intended semantics/usage. I've removed
  the description that task->thread.sve_state must have been allocated,
  as this is only necessary when task->thread.fp_type == FP_STATE_SVE,
  which itself implies that task->thread.sve_state must have been
  allocated.

* Rename the functions to more clearly indicate when/why they copy
  state:

  - sve_sync_to_fpsimd() => fpsimd_sync_from_effective_state()

  - sve_sync_from_fpsimd_zeropad => fpsimd_sync_to_effective_state_zeropad()

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20250508132644.1395904-7-mark.rutland@arm.com


Signed-off-by: Will Deacon <will@kernel.org>