fpu_lfpc_safe(9)

💅 fpu_lfpc_safe — Kernel API Manual

okay so bestie, welcome to fpu_lfpc_safe(9) — yes that's the manual name, yes it's a mouthful, no we're not renaming it, it's iconic as is 🎀

This is a kernel API page, so grab your strawberry drink 🍓 and let's get into it.

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✨ NAME ✨

fpu_lfpc_safe — literally just: load the floating point control register, but safely. The "safely" is doing a LOT of work here and we love her for it 💅

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🌸 SYNOPSIS 🌸

void fpu_lfpc_safe (unsigned int *fpc );

Copy it exactly, don't @ me. void return, one pointer argument, no drama 🦋

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🎀 ARGUMENTS 🎀

• fpc — the new value for the floating point control register. That's it. That's the whole argument. She's the main character of this function 👑

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💖 DESCRIPTION 💖

So here's the vibe: this function loads the floating point control register. Easy, right? NOT SO FAST bestie 😭

This may lead to an exception. Why? Because a saved value may have been modified by user space — and user space has hands, apparently:

• ptrace
• signal return
• kvm registers

...any of which could have turned that saved value into an invalid value. Rude, honestly. 🙈

And in such a case? The function sets the floating point control register to zero. That's the safety net, that's the whole point, periodt 🔥

no cap: the exception risk is real, and the zero fallback is the whole reason "safe" is in the name. Don't skip the details here 💀

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🫶 SEE ALSO 🫶

• Kernel file ./arch/s390/include/asm/fpu-insn.h — go read it, iykyk ⭐

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🌈 sign-off 🌈

That's the whole page, bestie! This one lives in ./arch/s390/include/asm, dated October 2026, and yes — it's fpu_lfpc_safe(9), the one and only 💜✨ Go forth and load those registers safely 🧚🌸