x86/fpu: Make 'no387' and 'nofxsr' command line options useful
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 1f999ab5 ("x86, xsave: Disable xsave in i387 emulation mode") to use `no387' on a CPU with XSAVE support. All this happens before console output. After hat, the next possible crash is in RAID6 detect code because MMX remained enabled. With a 3DNOW enabled config it will explode in memcpy() for instance due to kernel_fpu_begin() but this is unconditionally enabled. This is enough to boot a Debian Wheezy on a 32bit qemu "host" CPU which supports everything up to XSAVES, AVX2 without 3DNOW. Later, Debian increased the minimum requirements to i686 which means it does not boot userland atleast due to CMOV. After masking the additional features it still keeps SSE4A and 3DNOW* enabled (if present on the host) but those are unused in the kernel. Restrict `no387' and `nofxsr' otions to 32bit only. Add dependencies for FPU, FXSR to additionaly mask CMOV, MMX, XSAVE if FXSR or FPU is cleared. Reported-by:Vegard Nossum <vegard.nossum@oracle.com> Signed-off-by:
Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by:
Thomas Gleixner <tglx@linutronix.de> Link: https://lkml.kernel.org/r/20190703083247.57kjrmlxkai3vpw3@linutronix.de
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