[MIPS] Disallow CpU exception in kernel again.

The commit 4d40bff7110e9e1a97ff8c01bdd6350e9867cc10 ("Allow CpU
exception in kernel partially") was broken.  The commit was to fix
theoretical problem but broke usual case.  Revert it for now.
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/r2300_switch.S

@@ -49,7 +49,8 @@ LEAF(resume)
 #ifndef CONFIG_CPU_HAS_LLSC
 	sw      zero, ll_bit
 #endif
-	mfc0	t2, CP0_STATUS
+	mfc0	t1, CP0_STATUS
+	sw	t1, THREAD_STATUS(a0)
 	cpu_save_nonscratch a0
 	sw	ra, THREAD_REG31(a0)
 
@@ -59,8 +60,8 @@ LEAF(resume)
 	lw	t3, TASK_THREAD_INFO(a0)
 	lw	t0, TI_FLAGS(t3)
 	li	t1, _TIF_USEDFPU
-	and	t1, t0
-	beqz	t1, 1f
+	and	t2, t0, t1
+	beqz	t2, 1f
 	nor	t1, zero, t1
 
 	and	t0, t0, t1
@@ -73,13 +74,10 @@ LEAF(resume)
 	li	t1, ~ST0_CU1
 	and	t0, t0, t1
 	sw	t0, ST_OFF(t3)
-	/* clear thread_struct CU1 bit */
-	and	t2, t1
 
 	fpu_save_single a0, t0			# clobbers t0
 
 1:
-	sw	t2, THREAD_STATUS(a0)
 	/*
 	 * The order of restoring the registers takes care of the race
 	 * updating $28, $29 and kernelsp without disabling ints.

arch/mips/kernel/r4k_switch.S

@@ -48,7 +48,8 @@
 #ifndef CONFIG_CPU_HAS_LLSC
 	sw	zero, ll_bit
 #endif
-	mfc0	t2, CP0_STATUS
+	mfc0	t1, CP0_STATUS
+	LONG_S	t1, THREAD_STATUS(a0)
 	cpu_save_nonscratch a0
 	LONG_S	ra, THREAD_REG31(a0)
 
@@ -58,8 +59,8 @@
 	PTR_L	t3, TASK_THREAD_INFO(a0)
 	LONG_L	t0, TI_FLAGS(t3)
 	li	t1, _TIF_USEDFPU
-	and	t1, t0
-	beqz	t1, 1f
+	and	t2, t0, t1
+	beqz	t2, 1f
 	nor	t1, zero, t1
 
 	and	t0, t0, t1
@@ -72,13 +73,10 @@
 	li	t1, ~ST0_CU1
 	and	t0, t0, t1
 	LONG_S	t0, ST_OFF(t3)
-	/* clear thread_struct CU1 bit */
-	and	t2, t1
 
 	fpu_save_double a0 t0 t1		# c0_status passed in t0
 						# clobbers t1
 1:
-	LONG_S	t2, THREAD_STATUS(a0)
 
 	/*
 	 * The order of restoring the registers takes care of the race

arch/mips/kernel/signal.c

@@ -113,10 +113,10 @@ int setup_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc)
 		 * Save FPU state to signal context. Signal handler
 		 * will "inherit" current FPU state.
 		 */
+		preempt_disable();
 		own_fpu(1);
-		enable_fp_in_kernel();
 		err |= save_fp_context(sc);
-		disable_fp_in_kernel();
+		preempt_enable();
 	}
 	return err;
 }
@@ -148,7 +148,10 @@ check_and_restore_fp_context(struct sigcontext __user *sc)
 	err = sig = fpcsr_pending(&sc->sc_fpc_csr);
 	if (err > 0)
 		err = 0;
+	preempt_disable();
+	own_fpu(0);
 	err |= restore_fp_context(sc);
+	preempt_enable();
 	return err ?: sig;
 }
 
@@ -187,11 +190,8 @@ int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc)
 
 	if (used_math) {
 		/* restore fpu context if we have used it before */
-		own_fpu(0);
-		enable_fp_in_kernel();
 		if (!err)
 			err = check_and_restore_fp_context(sc);
-		disable_fp_in_kernel();
 	} else {
 		/* signal handler may have used FPU.  Give it up. */
 		lose_fpu(0);

arch/mips/kernel/signal32.c

@@ -209,10 +209,10 @@ static int setup_sigcontext32(struct pt_regs *regs,
 		 * Save FPU state to signal context.  Signal handler
 		 * will "inherit" current FPU state.
 		 */
+		preempt_disable();
 		own_fpu(1);
-		enable_fp_in_kernel();
 		err |= save_fp_context32(sc);
-		disable_fp_in_kernel();
+		preempt_enable();
 	}
 	return err;
 }
@@ -225,7 +225,10 @@ check_and_restore_fp_context32(struct sigcontext32 __user *sc)
 	err = sig = fpcsr_pending(&sc->sc_fpc_csr);
 	if (err > 0)
 		err = 0;
+	preempt_disable();
+	own_fpu(0);
 	err |= restore_fp_context32(sc);
+	preempt_enable();
 	return err ?: sig;
 }
 
@@ -261,11 +264,8 @@ static int restore_sigcontext32(struct pt_regs *regs,
 
 	if (used_math) {
 		/* restore fpu context if we have used it before */
-		own_fpu(0);
-		enable_fp_in_kernel();
 		if (!err)
 			err = check_and_restore_fp_context32(sc);
-		disable_fp_in_kernel();
 	} else {
 		/* signal handler may have used FPU.  Give it up. */
 		lose_fpu(0);

arch/mips/kernel/traps.c

@@ -757,11 +757,12 @@ asmlinkage void do_cpu(struct pt_regs *regs)
 {
 	unsigned int cpid;
 
+	die_if_kernel("do_cpu invoked from kernel context!", regs);
+
 	cpid = (regs->cp0_cause >> CAUSEB_CE) & 3;
 
 	switch (cpid) {
 	case 0:
-		die_if_kernel("do_cpu invoked from kernel context!", regs);
 		if (!cpu_has_llsc)
 			if (!simulate_llsc(regs))
 				return;
@@ -772,9 +773,6 @@ asmlinkage void do_cpu(struct pt_regs *regs)
 		break;
 
 	case 1:
-		if (!test_thread_flag(TIF_ALLOW_FP_IN_KERNEL))
-			die_if_kernel("do_cpu invoked from kernel context!",
-				      regs);
 		if (used_math())	/* Using the FPU again.  */
 			own_fpu(1);
 		else {			/* First time FPU user.  */
@@ -782,19 +780,7 @@ asmlinkage void do_cpu(struct pt_regs *regs)
 			set_used_math();
 		}
 
-		if (raw_cpu_has_fpu) {
-			if (test_thread_flag(TIF_ALLOW_FP_IN_KERNEL)) {
-				local_irq_disable();
-				if (cpu_has_fpu)
-					regs->cp0_status |= ST0_CU1;
-				/*
-				 * We must return without enabling
-				 * interrupts to ensure keep FPU
-				 * ownership until resume.
-				 */
-				return;
-			}
-		} else {
+		if (!raw_cpu_has_fpu) {
 			int sig;
 			sig = fpu_emulator_cop1Handler(regs,
 						&current->thread.fpu, 0);
@@ -836,7 +822,6 @@ asmlinkage void do_cpu(struct pt_regs *regs)
 
 	case 2:
 	case 3:
-		die_if_kernel("do_cpu invoked from kernel context!", regs);
 		break;
 	}
 

include/asm-mips/fpu.h

@@ -68,8 +68,6 @@ do {									\
 	/* We don't care about the c0 hazard here  */			\
 } while (0)
 
-#define __fpu_enabled()	(read_c0_status() & ST0_CU1)
-
 #define enable_fpu()							\
 do {									\
 	if (cpu_has_fpu)						\
@@ -162,18 +160,4 @@ static inline fpureg_t *get_fpu_regs(struct task_struct *tsk)
 	return tsk->thread.fpu.fpr;
 }
 
-static inline void enable_fp_in_kernel(void)
-{
-	set_thread_flag(TIF_ALLOW_FP_IN_KERNEL);
-	/* make sure CU1 and FPU ownership are consistent */
-	if (!__is_fpu_owner() && __fpu_enabled())
-		__disable_fpu();
-}
-
-static inline void disable_fp_in_kernel(void)
-{
-	BUG_ON(!__is_fpu_owner() && __fpu_enabled());
-	clear_thread_flag(TIF_ALLOW_FP_IN_KERNEL);
-}
-
 #endif /* _ASM_FPU_H */

include/asm-mips/thread_info.h

@@ -119,7 +119,6 @@ register struct thread_info *__current_thread_info __asm__("$28");
 #define TIF_POLLING_NRFLAG	17	/* true if poll_idle() is polling TIF_NEED_RESCHED */
 #define TIF_MEMDIE		18
 #define TIF_FREEZE		19
-#define TIF_ALLOW_FP_IN_KERNEL	20
 #define TIF_SYSCALL_TRACE	31	/* syscall trace active */
 
 #define _TIF_SYSCALL_TRACE	(1<<TIF_SYSCALL_TRACE)