MIPS: Implement ieee754 NAN2008 emulation mode

Implement ieee754 NAN2008 emulation mode.

When this mode is enabled, kernel will accept ELF file
compiled for both NaN 2008 and NaN legacy, but if hardware
does not have capability to match ELF's NaN mode, __own_fpu
will fail for corresponding thread and fpuemu will then kick
in.

This mode trade performance for correctness, while maintaining
support for both NaN mode regardless of hardware capability.
It is useful for multilib installation that have both types
of binary exist in system.
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

Documentation/admin-guide/kernel-parameters.txt

@@ -2022,7 +2022,7 @@
 			for the device. By default it is set to false (0).
 
 	ieee754=	[MIPS] Select IEEE Std 754 conformance mode
-			Format: { strict | legacy | 2008 | relaxed }
+			Format: { strict | legacy | 2008 | relaxed | emulated }
 			Default: strict
 
 			Choose which programs will be accepted for execution
@@ -2042,6 +2042,8 @@
 				by the FPU
 			relaxed	accept any binaries regardless of whether
 				supported by the FPU
+			emulated accept any binaries but enable FPU emulator
+				if binary mode is unsupported by the FPU.
 
 			The FPU emulator is always able to support both NaN
 			encodings, so if no FPU hardware is present or it has

arch/mips/include/asm/fpu.h

@@ -129,6 +129,18 @@ static inline int __own_fpu(void)
 	if (ret)
 		return ret;
 
+	if (current->thread.fpu.fcr31 & FPU_CSR_NAN2008) {
+		if (!cpu_has_nan_2008) {
+			ret = SIGFPE;
+			goto failed;
+		}
+	} else {
+		if (!cpu_has_nan_legacy) {
+			ret = SIGFPE;
+			goto failed;
+		}
+	}
+
 	KSTK_STATUS(current) |= ST0_CU1;
 	if (mode == FPU_64BIT || mode == FPU_HYBRID)
 		KSTK_STATUS(current) |= ST0_FR;
@@ -137,6 +149,9 @@ static inline int __own_fpu(void)
 
 	set_thread_flag(TIF_USEDFPU);
 	return 0;
+failed:
+	__disable_fpu();
+	return ret;
 }
 
 static inline int own_fpu_inatomic(int restore)

arch/mips/kernel/elf.c

@@ -318,6 +318,10 @@ void mips_set_personality_nan(struct arch_elf_state *state)
 	t->thread.fpu.fcr31 = c->fpu_csr31;
 	switch (state->nan_2008) {
 	case 0:
+		if (!(c->fpu_msk31 & FPU_CSR_NAN2008))
+			t->thread.fpu.fcr31 &= ~FPU_CSR_NAN2008;
+		if (!(c->fpu_msk31 & FPU_CSR_ABS2008))
+			t->thread.fpu.fcr31 &= ~FPU_CSR_ABS2008;
 		break;
 	case 1:
 		if (!(c->fpu_msk31 & FPU_CSR_NAN2008))

arch/mips/kernel/fpu-probe.c

@@ -144,7 +144,7 @@ static void cpu_set_fpu_2008(struct cpuinfo_mips *c)
  * IEEE 754 conformance mode to use.  Affects the NaN encoding and the
  * ABS.fmt/NEG.fmt execution mode.
  */
-static enum { STRICT, LEGACY, STD2008, RELAXED } ieee754 = STRICT;
+static enum { STRICT, EMULATED, LEGACY, STD2008, RELAXED } ieee754 = STRICT;
 
 /*
  * Set the IEEE 754 NaN encodings and the ABS.fmt/NEG.fmt execution modes
@@ -160,6 +160,7 @@ static void cpu_set_nofpu_2008(struct cpuinfo_mips *c)
 
 	switch (ieee754) {
 	case STRICT:
+	case EMULATED:
 		if (c->isa_level & (MIPS_CPU_ISA_M32R1 | MIPS_CPU_ISA_M64R1 |
 				    MIPS_CPU_ISA_M32R2 | MIPS_CPU_ISA_M64R2 |
 				    MIPS_CPU_ISA_M32R5 | MIPS_CPU_ISA_M64R5 |
@@ -204,6 +205,10 @@ static void cpu_set_nan_2008(struct cpuinfo_mips *c)
 		mips_use_nan_legacy = !cpu_has_nan_2008;
 		mips_use_nan_2008 = !!cpu_has_nan_2008;
 		break;
+	case EMULATED:
+		/* Pretend ABS2008/NAN2008 options are dynamic */
+		c->fpu_msk31 &= ~(FPU_CSR_NAN2008 | FPU_CSR_ABS2008);
+		fallthrough;
 	case RELAXED:
 		mips_use_nan_legacy = true;
 		mips_use_nan_2008 = true;
@@ -226,6 +231,8 @@ static int __init ieee754_setup(char *s)
 		return -1;
 	else if (!strcmp(s, "strict"))
 		ieee754 = STRICT;
+	else if (!strcmp(s, "emulated"))
+		ieee754 = EMULATED;
 	else if (!strcmp(s, "legacy"))
 		ieee754 = LEGACY;
 	else if (!strcmp(s, "2008"))