[PATCH 3.16 097/254] MIPS: Always clear FCSR cause bits after emulation

[Ben Hutchings <ben@xxxxxxxxxxxxxxx>]

3.16.55-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Maciej W. Rozycki" <macro@xxxxxxxxxxxxxx>

commit 443c44032a54f9acf027a8e688380fddc809bc19 upstream.

Clear any FCSR cause bits recorded in the saved FPU context after
emulation in all cases rather than in `do_fpe' only, so that any
unmasked IEEE 754 exception left from emulation does not cause a fatal
kernel-mode FPE hardware exception with the CTC1 instruction used by the
kernel to subsequently restore FCSR hardware from the saved FPU context.

Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxxxxxxxx>
Cc: linux-mips@xxxxxxxxxxxxxx
Patchwork: https://patchwork.linux-mips.org/patch/9704/
Signed-off-by: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
[bwh: Backported to 3.16: drop changes in mips-r2-to-r6-emul and simulate_fp()]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/arch/mips/kernel/traps.c
+++ b/arch/mips/kernel/traps.c
@@ -775,7 +775,7 @@ asmlinkage void do_fpe(struct pt_regs *r
 
 		/*
 		 * We can't allow the emulated instruction to leave any of
-		 * the cause bit set in $fcr31.
+		 * the cause bits set in $fcr31.
 		 */
 		current->thread.fpu.fcr31 &= ~FPU_CSR_ALL_X;
 
@@ -1292,6 +1292,13 @@ asmlinkage void do_cpu(struct pt_regs *r
 			sig = fpu_emulator_cop1Handler(regs,
 						       &current->thread.fpu,
 						       0, &fault_addr);
+
+			/*
+			 * We can't allow the emulated instruction to leave
+			 * any of the cause bits set in $fcr31.
+			 */
+			current->thread.fpu.fcr31 &= ~FPU_CSR_ALL_X;
+
 			if (!process_fpemu_return(sig, fault_addr) && !err)
 				mt_ase_fp_affinity();
 		}