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, 4 insertions, 2 deletions

diff --git a/arch/mips/math-emu/kernel_linkage.c b/arch/mips/math-emu/kernel_linkage.c
index 1c586575fe17..3aeae07ed5b8 100644
--- a/arch/mips/math-emu/kernel_linkage.c
+++ b/arch/mips/math-emu/kernel_linkage.c
@@ -89,8 +89,9 @@ int fpu_emulator_save_context32(struct sigcontext32 __user *sc)
 {
         int i;
         int err = 0;
+        int inc = test_thread_flag(TIF_32BIT_FPREGS) ? 2 : 1;
 
-        for (i = 0; i < 32; i+=2) {
+        for (i = 0; i < 32; i += inc) {
                 err |=
                     __put_user(current->thread.fpu.fpr[i], &sc->sc_fpregs[i]);
         }
@@ -103,8 +104,9 @@ int fpu_emulator_restore_context32(struct sigcontext32 __user *sc)
 {
         int i;
         int err = 0;
+        int inc = test_thread_flag(TIF_32BIT_FPREGS) ? 2 : 1;
 
-        for (i = 0; i < 32; i+=2) {
+        for (i = 0; i < 32; i += inc) {
                 err |=
                     __get_user(current->thread.fpu.fpr[i], &sc->sc_fpregs[i]);
         }