arm64/fpsimd: Don't corrupt FPMR when streaming mode changes

When the effective value of PSTATE.SM is changed from 0 to 1 or from 1
to 0 by any method, an entry or exit to/from streaming SVE mode is
performed, and hardware automatically resets a number of registers. As
of ARM DDI 0487 L.a, this means:

* All implemented bits of the SVE vector registers are set to zero.

* All implemented bits of the SVE predicate registers are set to zero.

* All implemented bits of FFR are set to zero, if FFR is implemented in
  the new mode.

* FPSR is set to 0x0000_0000_0800_009f.

* FPMR is set to 0, if FPMR is implemented.

Currently task_fpsimd_load() restores FPMR before restoring SVCR (which
is an accessor for PSTATE.{SM,ZA}), and so the restored value of FPMR
may be clobbered if the restored value of PSTATE.SM happens to differ
from the initial value of PSTATE.SM.

Fix this by moving the restore of FPMR later.

Note: this was originally posted as [1].

Fixes: 203f2b9 ("arm64/fpsimd: Support FEAT_FPMR")
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/linux-arm-kernel/20241204-arm64-sme-reenable-v2-2-bae87728251d@kernel.org/
[ Rutland: rewrite commit message ]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20250409164010.3480271-7-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: broonie

arch/arm64/kernel/fpsimd.c

@@ -359,9 +359,6 @@ static void task_fpsimd_load(void)
 	WARN_ON(preemptible());
 	WARN_ON(test_thread_flag(TIF_KERNEL_FPSTATE));
 
-	if (system_supports_fpmr())
-		write_sysreg_s(current->thread.uw.fpmr, SYS_FPMR);
-
 	if (system_supports_sve() || system_supports_sme()) {
 		switch (current->thread.fp_type) {
 		case FP_STATE_FPSIMD:
@@ -411,6 +408,9 @@ static void task_fpsimd_load(void)
 			restore_ffr = system_supports_fa64();
 	}
 
+	if (system_supports_fpmr())
+		write_sysreg_s(current->thread.uw.fpmr, SYS_FPMR);
+
 	if (restore_sve_regs) {
 		WARN_ON_ONCE(current->thread.fp_type != FP_STATE_SVE);
 		sve_load_state(sve_pffr(&current->thread),