linux-kernel - [PATCH v2 4/4] x86/fpu/64: Don't FNINIT in kernel_fpu

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Message-Id: <803d45a172a25314fdaec0a01aada8333d55df4e.1611077835.git.luto@kernel.org>
Date:   Tue, 19 Jan 2021 09:39:02 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Krzysztof Mazur <krzysiek@...lesie.net>,
        Krzysztof Olędzki <ole@....pl>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v2 4/4] 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@....pl>
Signed-off-by: Andy Lutomirski <luto@...nel.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 38f4936045ab..435bc59d539b 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
 }
 
 /*
-- 
2.29.2