On Fri, 23 Dec 2016, Yunqiang Su wrote: > > 3, kernel: the emulation when a float exception taken. > > The big problem is that Loongson use the same encode for (unfused) > madd.fmt to (fused) madd.fmt. We cannot trap this in kernel. Why is that a problem? Just add a setting like `cpu_has_fused_madd' to <asm/cpu-features.h>, switch processing in the emulator accordingly and initialise the setting early on in bootstrap as processor features are determined. We'd have to do it likewise for the R8000 MIPS IV CPU Richard referred to previously if we had full support for this processor (unlikely to happen). Cc-ing <linux-mips@xxxxxxxxxxxxxx> in case you want to discuss it further with the kernel developers; arguably it's a kernel bug already that the FPU as perceived by user software has different semantics on FPU Loongson hardware depending on whether or not the `nofpu' kernel parameter has been set (I don't know if denormals are handled by Loongson FPU hardware or trap with an Unimplemented Operation exception, but if the former, then this inconsistency applies for such data in the regular FPU mode as well). HTH, Maciej
