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, 6 insertions, 4 deletions

diff --git a/arch/mips/kernel/signal.c b/arch/mips/kernel/signal.c
index 2f285abc76d5..5199563c4403 100644
--- a/arch/mips/kernel/signal.c
+++ b/arch/mips/kernel/signal.c
@@ -71,8 +71,9 @@ static int protected_save_fp_context(struct sigcontext __user *sc)
 	int err;
 	while (1) {
 		lock_fpu_owner();
-		own_fpu_inatomic(1);
-		err = save_fp_context(sc); /* this might fail */
+		err = own_fpu_inatomic(1);
+		if (!err)
+			err = save_fp_context(sc); /* this might fail */
 		unlock_fpu_owner();
 		if (likely(!err))
 			break;
@@ -91,8 +92,9 @@ static int protected_restore_fp_context(struct sigcontext __user *sc)
 	int err, tmp __maybe_unused;
 	while (1) {
 		lock_fpu_owner();
-		own_fpu_inatomic(0);
-		err = restore_fp_context(sc); /* this might fail */
+		err = own_fpu_inatomic(0);
+		if (!err)
+			err = restore_fp_context(sc); /* this might fail */
 		unlock_fpu_owner();
 		if (likely(!err))
 			break;