MIPS: Add IEEE Std 754 conformance mode selection

Add an `ieee754=' kernel parameter to control IEEE Std 754 conformance
mode.

Use separate flags copied from the respective CPU feature flags, and
adjusted according to the conformance mode selected, to make binaries
requesting individual NaN encoding modes accepted or rejected as needed.
Update the initial setting for FCSR and, in the full FPU emulation mode,
its read-only mask accordingly.  Accept the mode selection requested for
legacy processors as well.

As with the EF_MIPS_NAN2008 ELF file header flag adjust both ABS2008 and
NAN2008 bits at the same time, to match the choice made for hardware
currently implemented.

Signed-off-by: Maciej W. Rozycki <macro@imgtec.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Fortune <Matthew.Fortune@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/11481/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

3 files changed, 107 insertions, 14 deletions

diff --git a/arch/mips/include/asm/elf.h b/arch/mips/include/asm/elf.h
index 3dba5d05830c..cefb7a596878 100644
--- a/arch/mips/include/asm/elf.h
+++ b/arch/mips/include/asm/elf.h
@@ -447,6 +447,10 @@ struct arch_elf_state {
 	.overall_fp_mode = -1,			\
 }
 
+/* Whether to accept legacy-NaN and 2008-NaN user binaries.  */
+extern bool mips_use_nan_legacy;
+extern bool mips_use_nan_2008;
+
 extern int arch_elf_pt_proc(void *ehdr, void *phdr, struct file *elf,
 			    bool is_interp, struct arch_elf_state *state);
 
diff --git a/arch/mips/kernel/cpu-probe.c b/arch/mips/kernel/cpu-probe.c
index 0aa61a95eb4b..b725b713b9f8 100644
--- a/arch/mips/kernel/cpu-probe.c
+++ b/arch/mips/kernel/cpu-probe.c
@@ -151,26 +151,109 @@ static void cpu_set_fpu_2008(struct cpuinfo_mips *c)
 }
 
 /*
- * Set the IEEE 754 NaN encodings and ABS.fmt/NEG.fmt execution modes
- * for the FPU emulator.  Clear the flags where required in case called
- * from `fpu_disable', to override details obtained from FPU hardware.
+ * 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;
+
+/*
+ * Set the IEEE 754 NaN encodings and the ABS.fmt/NEG.fmt execution modes
+ * to support by the FPU emulator according to the IEEE 754 conformance
+ * mode selected.  Note that "relaxed" straps the emulator so that it
+ * allows 2008-NaN binaries even for legacy processors.
  */
 static void cpu_set_nofpu_2008(struct cpuinfo_mips *c)
 {
+	c->options &= ~(MIPS_CPU_NAN_2008 | MIPS_CPU_NAN_LEGACY);
 	c->fpu_csr31 &= ~(FPU_CSR_ABS2008 | FPU_CSR_NAN2008);
-	if (c->isa_level & (MIPS_CPU_ISA_M32R1 | MIPS_CPU_ISA_M64R1 |
-			    MIPS_CPU_ISA_M32R2 | MIPS_CPU_ISA_M64R2 |
-			    MIPS_CPU_ISA_M32R6 | MIPS_CPU_ISA_M64R6)) {
-		c->options |= MIPS_CPU_NAN_2008 | MIPS_CPU_NAN_LEGACY;
-		c->fpu_msk31 &= ~(FPU_CSR_ABS2008 | FPU_CSR_NAN2008);
-	} else {
-		c->options &= ~MIPS_CPU_NAN_2008;
+	c->fpu_msk31 &= ~(FPU_CSR_ABS2008 | FPU_CSR_NAN2008);
+
+	switch (ieee754) {
+	case STRICT:
+		if (c->isa_level & (MIPS_CPU_ISA_M32R1 | MIPS_CPU_ISA_M64R1 |
+				    MIPS_CPU_ISA_M32R2 | MIPS_CPU_ISA_M64R2 |
+				    MIPS_CPU_ISA_M32R6 | MIPS_CPU_ISA_M64R6)) {
+			c->options |= MIPS_CPU_NAN_2008 | MIPS_CPU_NAN_LEGACY;
+		} else {
+			c->options |= MIPS_CPU_NAN_LEGACY;
+			c->fpu_msk31 |= FPU_CSR_ABS2008 | FPU_CSR_NAN2008;
+		}
+		break;
+	case LEGACY:
 		c->options |= MIPS_CPU_NAN_LEGACY;
 		c->fpu_msk31 |= FPU_CSR_ABS2008 | FPU_CSR_NAN2008;
+		break;
+	case STD2008:
+		c->options |= MIPS_CPU_NAN_2008;
+		c->fpu_csr31 |= FPU_CSR_ABS2008 | FPU_CSR_NAN2008;
+		c->fpu_msk31 |= FPU_CSR_ABS2008 | FPU_CSR_NAN2008;
+		break;
+	case RELAXED:
+		c->options |= MIPS_CPU_NAN_2008 | MIPS_CPU_NAN_LEGACY;
+		break;
+	}
+}
+
+/*
+ * Override the IEEE 754 NaN encoding and ABS.fmt/NEG.fmt execution mode
+ * according to the "ieee754=" parameter.
+ */
+static void cpu_set_nan_2008(struct cpuinfo_mips *c)
+{
+	switch (ieee754) {
+	case STRICT:
+		mips_use_nan_legacy = !!cpu_has_nan_legacy;
+		mips_use_nan_2008 = !!cpu_has_nan_2008;
+		break;
+	case LEGACY:
+		mips_use_nan_legacy = !!cpu_has_nan_legacy;
+		mips_use_nan_2008 = !cpu_has_nan_legacy;
+		break;
+	case STD2008:
+		mips_use_nan_legacy = !cpu_has_nan_2008;
+		mips_use_nan_2008 = !!cpu_has_nan_2008;
+		break;
+	case RELAXED:
+		mips_use_nan_legacy = true;
+		mips_use_nan_2008 = true;
+		break;
 	}
 }
 
 /*
+ * IEEE 754 NaN encoding and ABS.fmt/NEG.fmt execution mode override
+ * settings:
+ *
+ * strict:  accept binaries that request a NaN encoding supported by the FPU
+ * legacy:  only accept legacy-NaN binaries
+ * 2008:    only accept 2008-NaN binaries
+ * relaxed: accept any binaries regardless of whether supported by the FPU
+ */
+static int __init ieee754_setup(char *s)
+{
+	if (!s)
+		return -1;
+	else if (!strcmp(s, "strict"))
+		ieee754 = STRICT;
+	else if (!strcmp(s, "legacy"))
+		ieee754 = LEGACY;
+	else if (!strcmp(s, "2008"))
+		ieee754 = STD2008;
+	else if (!strcmp(s, "relaxed"))
+		ieee754 = RELAXED;
+	else
+		return -1;
+
+	if (!(boot_cpu_data.options & MIPS_CPU_FPU))
+		cpu_set_nofpu_2008(&boot_cpu_data);
+	cpu_set_nan_2008(&boot_cpu_data);
+
+	return 0;
+}
+
+early_param("ieee754", ieee754_setup);
+
+/*
  * Set the FIR feature flags for the FPU emulator.
  */
 static void cpu_set_nofpu_id(struct cpuinfo_mips *c)
@@ -212,6 +295,7 @@ static void cpu_set_fpu_opts(struct cpuinfo_mips *c)
 
 	cpu_set_fpu_fcsr_mask(c);
 	cpu_set_fpu_2008(c);
+	cpu_set_nan_2008(c);
 }
 
 /*
@@ -223,6 +307,7 @@ static void cpu_set_nofpu_opts(struct cpuinfo_mips *c)
 	c->fpu_msk31 = mips_nofpu_msk31;
 
 	cpu_set_nofpu_2008(c);
+	cpu_set_nan_2008(c);
 	cpu_set_nofpu_id(c);
 }
 
diff --git a/arch/mips/kernel/elf.c b/arch/mips/kernel/elf.c
index f36a261b275c..c3c234dc0c07 100644
--- a/arch/mips/kernel/elf.c
+++ b/arch/mips/kernel/elf.c
@@ -13,6 +13,10 @@
 
 #include <asm/cpu-info.h>
 
+/* Whether to accept legacy-NaN and 2008-NaN user binaries.  */
+bool mips_use_nan_legacy;
+bool mips_use_nan_2008;
+
 /* FPU modes */
 enum {
 	FP_FRE,
@@ -150,16 +154,16 @@ int arch_check_elf(void *_ehdr, bool has_interpreter, void *_interp_ehdr,
 	flags = elf32 ? ehdr->e32.e_flags : ehdr->e64.e_flags;
 
 	/*
-	 * Determine the NaN personality, reject the binary if no hardware
-	 * support.  Also ensure that any interpreter matches the executable.
+	 * Determine the NaN personality, reject the binary if not allowed.
+	 * Also ensure that any interpreter matches the executable.
 	 */
 	if (flags & EF_MIPS_NAN2008) {
-		if (cpu_has_nan_2008)
+		if (mips_use_nan_2008)
 			state->nan_2008 = 1;
 		else
 			return -ENOEXEC;
 	} else {
-		if (cpu_has_nan_legacy)
+		if (mips_use_nan_legacy)
 			state->nan_2008 = 0;
 		else
 			return -ENOEXEC;