s390/nmi: improve revalidation of fpu / vector registers

The machine check handler will do one of two things if the floating-point
control, a floating point register or a vector register can not be
revalidated:
1) if the PSW indicates user mode the process is terminated
2) if the PSW indicates kernel mode the system is stopped

To unconditionally stop the system for 2) is incorrect.

There are three possible outcomes if the floating-point control, a
floating point register or a vector registers can not be revalidated:
1) The kernel is inside a kernel_fpu_begin/kernel_fpu_end block and
needs the register. The system is stopped.
2) No active kernel_fpu_begin/kernel_fpu_end block and the CIF_CPU bit
is not set. The user space process needs the register and is killed.
3) No active kernel_fpu_begin/kernel_fpu_end block and the CIF_FPU bit
is set. Neither the kernel nor the user space process needs the
lost register. Just revalidate it and continue.

Signed-off-by: Martin Schwidefsky <[e-mail]>

arch/s390/kernel/nmi.c

···
  * returns 0 if all registers could be validated
  * returns 1 otherwise
  */
-static int notrace s390_validate_registers(union mci mci)
+static int notrace s390_validate_registers(union mci mci, int umode)
 {
 	int kill_task;
 	u64 zero;
···
 	if (!mci.gr) {
 		/*
 		 * General purpose registers couldn't be restored and have
-		 * unknown contents. Process needs to be terminated.
+		 * unknown contents. Stop system or terminate process.
 		 */
+		if (!umode)
+			s390_handle_damage();
 		kill_task = 1;
 	}
 	if (!mci.fp) {
 		/*
-		 * Floating point registers can't be restored and
-		 * therefore the process needs to be terminated.
+		 * Floating point registers can't be restored. If the
+		 * kernel currently uses floating point registers the
+		 * system is stopped. If the process has its floating
+		 * pointer registers loaded it is terminated.
+		 * Otherwise just revalidate the registers.
 		 */
-		kill_task = 1;
+		if (S390_lowcore.fpu_flags & KERNEL_VXR_V0V7)
+			s390_handle_damage();
+		if (!test_cpu_flag(CIF_FPU))
+			kill_task = 1;
 	}
 	fpt_save_area = &S390_lowcore.floating_pt_save_area;
 	fpt_creg_save_area = &S390_lowcore.fpt_creg_save_area;
 	if (!mci.fc) {
 		/*
 		 * Floating point control register can't be restored.
-		 * Task will be terminated.
+		 * If the kernel currently uses the floating pointer
+		 * registers and needs the FPC register the system is
+		 * stopped. If the process has its floating pointer
+		 * registers loaded it is terminated. Otherwiese the
+		 * FPC is just revalidated.
 		 */
+		if (S390_lowcore.fpu_flags & KERNEL_FPC)
+			s390_handle_damage();
 		asm volatile("lfpc 0(%0)" : : "a" (&zero), "m" (zero));
-		kill_task = 1;
+		if (!test_cpu_flag(CIF_FPU))
+			kill_task = 1;
 	} else
 		asm volatile("lfpc 0(%0)" : : "a" (fpt_creg_save_area));
 
···
 
 		if (!mci.vr) {
 			/*
-			 * Vector registers can't be restored and therefore
-			 * the process needs to be terminated.
+			 * Vector registers can't be restored. If the kernel
+			 * currently uses vector registers the system is
+			 * stopped. If the process has its vector registers
+			 * loaded it is terminated. Otherwise just revalidate
+			 * the registers.
 			 */
-			kill_task = 1;
+			if (S390_lowcore.fpu_flags & KERNEL_VXR)
+				s390_handle_damage();
+			if (!test_cpu_flag(CIF_FPU))
+				kill_task = 1;
 		}
 		cr0.val = S390_lowcore.cregs_save_area[0];
 		cr0.afp = cr0.vx = 1;
···
 	struct mcck_struct *mcck;
 	unsigned long long tmp;
 	union mci mci;
-	int umode;
 
 	nmi_enter();
 	inc_irq_stat(NMI_NMI);
 	mci.val = S390_lowcore.mcck_interruption_code;
 	mcck = this_cpu_ptr(&cpu_mcck);
-	umode = user_mode(regs);
 
 	if (mci.sd) {
 		/* System damage -> stopping machine */
···
 			s390_handle_damage();
 		}
 	}
-	if (s390_validate_registers(mci)) {
-		if (umode) {
-			/*
-			 * Couldn't restore all register contents while in
-			 * user mode -> mark task for termination.
-			 */
-			mcck->kill_task = 1;
-			mcck->mcck_code = mci.val;
-			set_cpu_flag(CIF_MCCK_PENDING);
-		} else {
-			/*
-			 * Couldn't restore all register contents while in
-			 * kernel mode -> stopping machine.
-			 */
-			s390_handle_damage();
-		}
+	if (s390_validate_registers(mci, user_mode(regs))) {
+		/*
+		 * Couldn't restore all register contents for the
+		 * user space process -> mark task for termination.
+		 */
+		mcck->kill_task = 1;
+		mcck->mcck_code = mci.val;
+		set_cpu_flag(CIF_MCCK_PENDING);
 	}
 	if (mci.cd) {
 		/* Timing facility damage */