From 49200d17d27d5cc1aede2d1bb2a78dbfc1563e65 Mon Sep 17 00:00:00 2001
From: Andy Lutomirski <luto@kernel.org>
Date: Wed, 20 Jan 2021 21:09:51 -0800
Subject: [PATCH] x86/fpu/64: Don't FNINIT in kernel_fpu_begin()
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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
---
arch/x86/include/asm/fpu/api.h | 12 ++++++++++++
1 file changed, 12 insertions(+)
diff --git a/arch/x86/include/asm/fpu/api.h b/arch/x86/include/asm/fpu/api.h
index 67a4f1cb2aac..ed33a14188f6 100644
--- 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
}
/*
--
2.31.1
