MIPS: preserve scalar FP CSR when switching vector context

Switching the vector context implicitly saves & restores the state of
the aliased scalar FP data registers, however the scalar FP control
& status register is distinct from the MSA control & status register.
In order to allow scalar FP to function correctly in programs using
MSA, the scalar CSR needs to be saved & restored along with the MSA
vector context.
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7301/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/r4k_switch.S

@@ -64,8 +64,10 @@
 	/* Check whether we're saving scalar or vector context. */
 	bgtz	a3, 1f
 
-	/* Save 128b MSA vector context. */
+	/* Save 128b MSA vector context + scalar FP control & status. */
+	cfc1	t1, fcr31
 	msa_save_all	a0
+	sw	t1, THREAD_FCR31(a0)
 	b	2f
 
 1:	/* Save 32b/64b scalar FP context. */

arch/mips/kernel/traps.c

@@ -1154,6 +1154,11 @@ static int enable_restore_fp_context(int msa)
 
 	/* We need to restore the vector context. */
 	restore_msa(current);
+
+	/* Restore the scalar FP control & status register */
+	if (!was_fpu_owner)
+		asm volatile("ctc1 %0, $31" : : "r"(current->thread.fpu.fcr31));
+
 	return 0;
 }