x86/fpu/64: Don't FNINIT in kernel_fpu_begin()

The remaining callers of kernel_fpu_begin() in 64-bit kernels don't use 387
instructions, so there's no need to sanitize the FPU state.  Skip it to get
most of the performance we lost back.

Reported-by: Krzysztof Olędzki <ole@ans.pl>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/57f8841ccbf9f3c25a23196c888f5f6ec5887577.1611205691.git.luto@kernel.org

diff --git a/arch/x86/include/asm/fpu/api.h b/arch/x86/include/asm/fpu/api.h
index 67a4f1cb2aac58cf196fd4cd645b9736a0d76aaf..ed33a14188f6642bea84771f65792f8dd457905f 100644 (file)
--- a/arch/x86/include/asm/fpu/api.h
+++ b/arch/x86/include/asm/fpu/api.h
@@ -32,7 +32,19 @@ extern void fpregs_mark_activate(void);
 /* Code that is unaware of kernel_fpu_begin_mask() can use this */
 static inline void kernel_fpu_begin(void)
 {
+#ifdef CONFIG_X86_64
+       /*
+        * Any 64-bit code that uses 387 instructions must explicitly request
+        * KFPU_387.
+        */
+       kernel_fpu_begin_mask(KFPU_MXCSR);
+#else
+       /*
+        * 32-bit kernel code may use 387 operations as well as SSE2, etc,
+        * as long as it checks that the CPU has the required capability.
+        */
        kernel_fpu_begin_mask(KFPU_387 | KFPU_MXCSR);
+#endif
 }
 
 /*