arm64/fpsimd: signal: Use SMSTOP behaviour in setup_return()

Historically the behaviour of setup_return() was nondeterministic,
depending on whether the task's FSIMD/SVE/SME state happened to be live.
We fixed most of that in commit:

  929fa99 ("arm64/fpsimd: signal: Always save+flush state early")

... but we didn't decide on how clearing PSTATE.SM should behave, and left a
TODO comment to that effect.

Use the new task_smstop_sm() helper to make this behave as if an SMSTOP
instruction was used to exit streaming mode. This would have been the
most common behaviour prior to the commit above.

Fixes: 40a8e87 ("arm64/sme: Disable ZA and streaming mode when handling signals")
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/20250508132644.1395904-10-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

Author: mrutland-arm

arch/arm64/kernel/signal.c

@@ -1476,22 +1476,8 @@ static int setup_return(struct pt_regs *regs, struct ksignal *ksig,
 
 	/* Signal handlers are invoked with ZA and streaming mode disabled */
 	if (system_supports_sme()) {
-		/*
-		 * If we were in streaming mode the saved register
-		 * state was SVE but we will exit SM and use the
-		 * FPSIMD register state.
-		 *
-		 * TODO: decide if this should behave as SMSTOP (e.g. reset
-		 * FPSR + FPMR), or whether this should only clear the scalable
-		 * registers + ZA state.
-		 */
-		if (current->thread.svcr & SVCR_SM_MASK) {
-			memset(&current->thread.uw.fpsimd_state, 0,
-			       sizeof(current->thread.uw.fpsimd_state));
-			current->thread.fp_type = FP_STATE_FPSIMD;
-		}
-
-		current->thread.svcr &= ~(SVCR_ZA_MASK | SVCR_SM_MASK);
+		task_smstop_sm(current);
+		current->thread.svcr &= ~SVCR_ZA_MASK;
 		write_sysreg_s(0, SYS_TPIDR2_EL0);
 	}