MIPS: Support for 64-bit FP with O32 binaries

CPUs implementing MIPS32 R2 may include a 64-bit FPU, just as MIPS64 CPUs
do. In order to preserve backwards compatibility a 64-bit FPU will act
like a 32-bit FPU (by accessing doubles from the least significant 32
bits of an even-odd pair of FP registers) when the Status.FR bit is
zero, again just like a mips64 CPU. The standard O32 ABI is defined
expecting a 32-bit FPU, however recent toolchains support use of a
64-bit FPU from an O32 MIPS32 executable. When an ELF executable is
built to use a 64-bit FPU a new flag (EF_MIPS_FP64) is set in the ELF
header.

With this patch the kernel will check the EF_MIPS_FP64 flag when
executing an O32 binary, and set Status.FR accordingly. The addition
of O32 64-bit FP support lessens the opportunity for optimisation in
the FPU emulator, so a CONFIG_MIPS_O32_FP64_SUPPORT Kconfig option is
introduced to allow this support to be disabled for those that don't
require it.

Inspired by an earlier patch by Leonid Yegoshin, but implemented more
cleanly & correctly.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Paul Burton <paul.burton@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/6154/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 28 insertions, 3 deletions

diff --git a/arch/mips/include/asm/elf.h b/arch/mips/include/asm/elf.h
index a66359ef4ece..d4144056e928 100644
--- a/arch/mips/include/asm/elf.h
+++ b/arch/mips/include/asm/elf.h
@@ -36,6 +36,7 @@
 #define EF_MIPS_ABI2		0x00000020
 #define EF_MIPS_OPTIONS_FIRST	0x00000080
 #define EF_MIPS_32BITMODE	0x00000100
+#define EF_MIPS_FP64		0x00000200
 #define EF_MIPS_ABI		0x0000f000
 #define EF_MIPS_ARCH		0xf0000000
 
@@ -176,6 +177,18 @@ typedef elf_fpreg_t elf_fpregset_t[ELF_NFPREG];
 #ifdef CONFIG_32BIT
 
 /*
+ * In order to be sure that we don't attempt to execute an O32 binary which
+ * requires 64 bit FP (FR=1) on a system which does not support it we refuse
+ * to execute any binary which has bits specified by the following macro set
+ * in its ELF header flags.
+ */
+#ifdef CONFIG_MIPS_O32_FP64_SUPPORT
+# define __MIPS_O32_FP64_MUST_BE_ZERO	0
+#else
+# define __MIPS_O32_FP64_MUST_BE_ZERO	EF_MIPS_FP64
+#endif
+
+/*
  * This is used to ensure we don't load something for the wrong architecture.
  */
 #define elf_check_arch(hdr)						\
@@ -192,6 +205,8 @@ typedef elf_fpreg_t elf_fpregset_t[ELF_NFPREG];
 	if (((__h->e_flags & EF_MIPS_ABI) != 0) &&			\
 	    ((__h->e_flags & EF_MIPS_ABI) != EF_MIPS_ABI_O32))		\
 		__res = 0;						\
+	if (__h->e_flags & __MIPS_O32_FP64_MUST_BE_ZERO)		\
+		__res = 0;						\
 									\
 	__res;								\
 })
@@ -249,6 +264,11 @@ extern struct mips_abi mips_abi_n32;
 
 #define SET_PERSONALITY(ex)						\
 do {									\
+	if ((ex).e_flags & EF_MIPS_FP64)				\
+		clear_thread_flag(TIF_32BIT_FPREGS);			\
+	else								\
+		set_thread_flag(TIF_32BIT_FPREGS);			\
+									\
 	if (personality(current->personality) != PER_LINUX)		\
 		set_personality(PER_LINUX);				\
 									\
@@ -271,14 +291,18 @@ do {									\
 #endif
 
 #ifdef CONFIG_MIPS32_O32
-#define __SET_PERSONALITY32_O32()					\
+#define __SET_PERSONALITY32_O32(ex)					\
 	do {								\
 		set_thread_flag(TIF_32BIT_REGS);			\
 		set_thread_flag(TIF_32BIT_ADDR);			\
+									\
+		if (!((ex).e_flags & EF_MIPS_FP64))			\
+			set_thread_flag(TIF_32BIT_FPREGS);		\
+									\
 		current->thread.abi = &mips_abi_32;			\
 	} while (0)
 #else
-#define __SET_PERSONALITY32_O32()					\
+#define __SET_PERSONALITY32_O32(ex)					\
 	do { } while (0)
 #endif
 
@@ -289,7 +313,7 @@ do {									\
 	     ((ex).e_flags & EF_MIPS_ABI) == 0)				\
 		__SET_PERSONALITY32_N32();				\
 	else								\
-		__SET_PERSONALITY32_O32();				\
+		__SET_PERSONALITY32_O32(ex);                            \
 } while (0)
 #else
 #define __SET_PERSONALITY32(ex) do { } while (0)
@@ -300,6 +324,7 @@ do {									\
 	unsigned int p;							\
 									\
 	clear_thread_flag(TIF_32BIT_REGS);				\
+	clear_thread_flag(TIF_32BIT_FPREGS);				\
 	clear_thread_flag(TIF_32BIT_ADDR);				\
 									\
 	if ((ex).e_ident[EI_CLASS] == ELFCLASS32)			\