[PATCH] x86/lib: don't use MMX before FPU initialization

[Krzysztof Mazur]

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: <stable@xxxxxxxxxxxxxxx> # 5.8+
Signed-off-by: Krzysztof Mazur <krzysiek@xxxxxxxxxxxx>
---
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