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Linux/LegoOS x86 Floating Point Unit

Version History
Date Description
Jan 9, 2021 repolished after reading the why mmap is faster syscall post. Indeed, the difference is that mmap is using user-level AVX-aided memmove while kernel cannot. This reminded me of this post so I decided to move it to here.
Feb 22, 2018 Initial Version

This blog documents how kernel is dealing with x86 FPU at a high level.


FPU is heavily used by user level code, but not kernel. You may not use it directly, but glibc is using it all over the place, e.g. the strcmp, memcpy. x86 FPU is really a super complex technology designed by Intel. Of course its performance is good and also widely used, but the legacy compatible feature? Hmm, not so yummy.

Without Ingo Molnar’s x86 FPU code rewrite, there is no way for me to easily understand it. In 2019, the FPU code received another huge improvement (patch).

The current x86 FPU code is well-written. Even though I don’t understand some of the low-level code, I do enjoy reading it. The naming convention, the code organization, the file organization, the header files, it is a nice piece of art.

Below I will briefly list kernel subsystems that use FPU. My understanding is based on code before the 2019 FPU patch, so some facts may have changed already.

Boot

FPU detection and init happen during early boot. The struct fpu is a dynamically-sized structure. Its size depends on what features the underlying CPU support. Since struct fpu is part of struct task_struct, that implies task_struct is dynamically-sized as well (task_struct -> thread_struct -> fpu). The cpu_init() will also callback to init its local FPU.

Context Switch

FPU consists of a huge amount of registers. Each thread will have its own FPU context. However, the CPU itself will not save or restore any FPU registers automatically, it is software’s duty to save and restore FPU context properly. And FPU context/registers are saved into struct fpu.

Thus whenever we switch task, we also need to switch FPU context (note: not always, it is optional, kernel is using a lazy switching trick). Code:

__visible struct task_struct *
__switch_to(struct task_struct *prev_p, struct task_struct *next_p)
{
        ..
        fpu_switch = switch_fpu_prepare(prev_fpu, next_fpu, cpu);
        ..
        switch_fpu_finish(next_fpu, fpu_switch);
        ..
}

SYSCALL

  • fork() and clone(): When a new thread or process is created, the FPU context is copied from the calling thread.
  • execve(): during this syscall, the FPU context will be cleared.
  • exit(): When a thread exit, FPU will do cleanup based on whether eagerfpu or lazyfpu is used.

Exceptions

Like the device not available exception, which may be triggered if lazyfpu is used. The do_simd_exception() and do_coprocessor_error() are some math related exceptions.

Signal

Kernel needs to setup a sigframe for user level signal handlers. sigframe is a contiguous stack memory consists of the general purpose registers AND FPU registers. So signal handling part has to call back to FPU code to setup and copy the FPU registers to the in stack sigframe.

Signal handling is another beast.

Thoughts

Compatibility is a heavy thing to carry. But it is also a nice thing for marketing. No one can deny the success of Intel on its backward compatibility. Bad for low-level system developers.

References

  1. https://unix.stackexchange.com/questions/475956/why-can-the-kernel-not-use-sse-avx-registers-and-instructions
  2. 2019 FPU patch: https://lkml.org/lkml/2019/4/3/877

Last update: October 23, 2021

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