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, 5 insertions, 5 deletions

diff --git a/arch/mips/kernel/traps.c b/arch/mips/kernel/traps.c
index f9c8746be8d6..f40f688276c2 100644
--- a/arch/mips/kernel/traps.c
+++ b/arch/mips/kernel/traps.c
@@ -1080,7 +1080,7 @@ asmlinkage void do_cpu(struct pt_regs *regs)
 	unsigned long old_epc, old31;
 	unsigned int opcode;
 	unsigned int cpid;
-	int status;
+	int status, err;
 	unsigned long __maybe_unused flags;
 
 	prev_state = exception_enter();
@@ -1153,19 +1153,19 @@ asmlinkage void do_cpu(struct pt_regs *regs)
 
 	case 1:
 		if (used_math())	/* Using the FPU again.	 */
-			own_fpu(1);
+			err = own_fpu(1);
 		else {			/* First time FPU user.	 */
-			init_fpu();
+			err = init_fpu();
 			set_used_math();
 		}
 
-		if (!raw_cpu_has_fpu) {
+		if (!raw_cpu_has_fpu || err) {
 			int sig;
 			void __user *fault_addr = NULL;
 			sig = fpu_emulator_cop1Handler(regs,
 						       &current->thread.fpu,
 						       0, &fault_addr);
-			if (!process_fpemu_return(sig, fault_addr))
+			if (!process_fpemu_return(sig, fault_addr) && !err)
 				mt_ase_fp_affinity();
 		}