On Mon, Apr 7, 2014 at 3:53 PM, Paul Burton <paul.burton@xxxxxxxxxx> wrote:
> On Mon, Apr 07, 2014 at 12:57:04PM +0200, Manuel Lauss wrote:
>> This small patch makes the MIPS FPU emulator optional. The kernel
>> kills float-users on systems without a hardware FPU by sending a SIGILL.
>
> One issue with this is that if someone runs a kernel with the FPU
> emulator disabled on hardware that has an FPU, they're likely to hit
> seemingly odd behaviour where FP works just fine until they hit a
> condition the hardware doesn't support. To make it clear that using FP
> without the emulator is a bad idea, perhaps it would be safer to disable
> FP entirely rather than only the emulator? Then userland can die the
> first time it uses FP instead of when it happens to operate on a
> denormal.
Very good point, I understand.
How about this addon-patch? I don't want to sprinkle the whole codebase
with #ifdef MIPS_FPU_SUPPORT lines.
Untested, since I don't have any hardware with FPU.
> Unless there are FPUs which never generate an unimplemented operation
> exception, in which case perhaps more Kconfig is needed to identify such
> systems & allow the emulator to be disabled for those only.
I'd rather keep the simple patch I sent, and maybe hide the option behind
CONFIG_EXPERT so only people who want to save space and know what
they're doing can disable it (e.g. OpenWRT).
diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 3924396..52de5b8 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -2482,19 +2482,16 @@ config MIPS_O32_FP64_SUPPORT
If unsure, say N.
-config MIPS_FPU_EMULATOR
- bool "MIPS FPU Emulator"
+config MIPS_FPU_SUPPORT
+ bool "MIPS FPU Support"
default y
help
- This option lets you disable the built-in MIPS FPU (Coprocessor 1)
- emulator, which handles floating-point instructions on processors
- without a hardware FPU. It is generally a good idea to keep the
- emulator built-in, unless you are perfectly sure you have a
- complete soft-float environment. With the emulator disabled, all
- users of float operations will be killed with an illegal instr-
- uction exception.
+ Enable support for floating point math, be it hardware FPU or the
+ kernels' FPU emulator. With this option disabled, any user of
+ float math will be killed by illegal instruction exception,
+ regardless of the availability of hardware floating point support.
- Say Y, please.
+ Say Y, unless you have a pure soft-float userspace.
config USE_OF
bool
diff --git a/arch/mips/include/asm/fpu.h b/arch/mips/include/asm/fpu.h
index c5203bb..ed68719 100644
--- a/arch/mips/include/asm/fpu.h
+++ b/arch/mips/include/asm/fpu.h
@@ -156,13 +156,14 @@ static inline int init_fpu(void)
int ret = 0;
preempt_disable();
- if (cpu_has_fpu) {
- ret = __own_fpu();
- if (!ret)
- _init_fpu();
- } else if (IS_ENABLED(CONFIG_MIPS_FPU_EMULATOR))
- fpu_emulator_init_fpu();
- else
+ if (IS_ENABLED(CONFIG_MIPS_FPU_SUPPORT)) {
+ if (cpu_has_fpu) {
+ ret = __own_fpu();
+ if (!ret)
+ _init_fpu();
+ } else
+ fpu_emulator_init_fpu();
+ } else
ret = SIGILL;
preempt_enable();
Thanks,
Mano
