When enabled, the MMX 3DNow! optimized memcpy() is used very early,
even before FPU is initialized. It worked fine, but commit
7ad816762f9bf89e940e618ea40c43138b479e10 ("x86/fpu: Reset MXCSR
to default in kernel_fpu_begin()") broke that. After that commit
the kernel crashes just after "Booting the kernel." message.
It affects all kernels with CONFIG_X86_USE_3DNOW=y (enabled when
some AMD/Cyrix processors are selected). So, enable MMX 3DNow!
optimized memcpy() later.Cc: <[email protected]> # 5.8+ Signed-off-by: Krzysztof Mazur <[email protected]> --- Hi, this patch fixes a kernel crash during boot observed on AMD Athlon XP (with CONFIG_MK7=y). The static key adds 5 byte NOP in the "fast" path. The 3DNow! usage can be enabled earlier, but arch_initcall should be ok. The static_cpu_has() does not work because the kernel with CONFIG_X86_USE_3DNOW=y assumes that 3DNOW! is available, so a static key is used instead. "Alternatives" should also work, as long they not assume that required features (REQUIRED_MASK*) are available early. Similar bugs are possible. For easier debugging, maybe kernel_fpu_begin() should catch such cases and print something? Thanks, Krzysiek arch/x86/lib/mmx_32.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/arch/x86/lib/mmx_32.c b/arch/x86/lib/mmx_32.c index 4321fa02e18d..375bf0798993 100644 --- a/arch/x86/lib/mmx_32.c +++ b/arch/x86/lib/mmx_32.c @@ -26,12 +26,14 @@ #include <asm/fpu/api.h> #include <asm/asm.h> +static __ro_after_init DEFINE_STATIC_KEY_FALSE(use_mmx); + void *_mmx_memcpy(void *to, const void *from, size_t len) { void *p; int i; - if (unlikely(in_interrupt())) + if (unlikely(in_interrupt()) || !static_branch_likely(&use_mmx)) return __memcpy(to, from, len); p = to; @@ -376,3 +378,11 @@ void mmx_copy_page(void *to, void *from) fast_copy_page(to, from); } EXPORT_SYMBOL(mmx_copy_page); + +static int __init mmx_32_init(void) +{ + static_branch_enable(&use_mmx); + return 0; +} + +arch_initcall(mmx_32_init); -- 2.27.0.rc1.207.gb85828341f
