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Re: Loongson FPU Bug


On Fri, Feb 3, 2023 at 2:37 PM Florian Lohoff <f@zz.de> wrote:
> Hi,
> does anyone have a document describing the Loongson FPU bug?
> I am currently rebuilding stretch++ for --march=mips2 as all machines i
> did care for (Cobald Cube, Siemens RM200, R4k Decstation) are
> unsupported since jessie. I am building on a Loongson and i get bitten
> by the FPU bug at random occasions.

I'm not sure which version of Loongson CPU you are using, anyway these
are several of them:

1. madd.X/msub.X: they are fused implementations while others are
non-fused, disabled by default in Debian
2. EI/DI: only affect privileged mode so that the kernel needs to be
compiled with mips64r1 (which I believe it is default upstream)
3. swxc1: would report  illegal hardware instruction on some hardware,
and some octean boards will report SIGBUS
4. FR0 FPU emulation mode is not architecturally valid, but this does
not likely affect you since you are not using mips32r1/r2.


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