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The command line option `no387' is designed to disable the FPU
entirely. This only 'works' with CONFIG_MATH_EMULATION enabled.
But on 64bit this cannot work because user space expects SSE to work which
required basic FPU support. MATH_EMULATION does not help because SSE is not
emulated.
The command line option `nofxsr' should also be limited to 32bit because
FXSR is part of the required flags on 64bit so turning it off is not
possible.
Clearing X86_FEATURE_FPU without emulation enabled will not work anyway and
hang in fpu__init_system_early_generic() before the console is enabled.
Setting additioal dependencies, ensures that the CPU still boots on a
modern CPU. Otherwise, dropping FPU will leave FXSR enabled causing the
kernel to crash early in fpu__init_system_mxcsr().
With XSAVE support it will crash in fpu__init_cpu_xstate(). The problem is
that xsetbv() with XMM set and SSE cleared is not allowed. That means
XSAVE has to be disabled. The XSAVE support is disabled in
fpu__init_system_xstate_size_legacy() but it is too late. It can be
removed, it has been added in commit
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| .. | ||
| boot | ||
| configs | ||
| crypto | ||
| entry | ||
| events | ||
| hyperv | ||
| ia32 | ||
| include | ||
| kernel | ||
| kvm | ||
| lib | ||
| math-emu | ||
| mm | ||
| net | ||
| oprofile | ||
| pci | ||
| platform | ||
| power | ||
| purgatory | ||
| ras | ||
| realmode | ||
| tools | ||
| um | ||
| video | ||
| xen | ||
| .gitignore | ||
| Kbuild | ||
| Kconfig | ||
| Kconfig.cpu | ||
| Kconfig.debug | ||
| Makefile | ||
| Makefile_32.cpu | ||
| Makefile.um | ||