linux-kernel - [PATCH 4/4] x86/cpu: Provide a config option to disable static_cpu

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Date:	Tue,  1 Dec 2015 16:00:23 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	X86 ML <x86@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Josh Triplett <josh@...htriplett.org>
Subject: [PATCH 4/4] x86/cpu: Provide a config option to disable static_cpu_has

From: Borislav Petkov <bp@...e.de>

This brings .text savings of about ~1.6K when building a tinyconfig. It
is off by default so nothing changes for the default.

Kconfig help text from Josh.

Signed-off-by: Borislav Petkov <bp@...e.de>
Cc: Josh Triplett <josh@...htriplett.org>
---
 arch/x86/Kconfig                  | 11 +++++++++++
 arch/x86/include/asm/cpufeature.h |  2 +-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index db3622f22b61..a2abc2fb6970 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -349,6 +349,17 @@ config X86_FEATURE_NAMES
 
 	  If in doubt, say Y.
 
+config X86_FAST_FEATURE_TESTS
+	bool "Fast CPU feature tests" if EMBEDDED
+	default y
+	---help---
+	  Some fast-paths in the kernel depend on the capabilities of the CPU.
+	  Say Y here for the kernel to patch in the appropriate code at runtime
+	  based on the capabilities of the CPU. The infrastructure for patching
+	  code at runtime takes up some additional space; space-constrained
+	  embedded systems may wish to say N here to produce smaller, slightly
+	  slower code.
+
 config X86_X2APIC
 	bool "Support x2apic"
 	depends on X86_LOCAL_APIC && X86_64 && (IRQ_REMAP || HYPERVISOR_GUEST)
diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 144b042c0872..43e144474043 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -409,7 +409,7 @@ extern const char * const x86_bug_flags[NBUGINTS*32];
  * fast paths and boot_cpu_has() otherwise!
  */
 
-#if __GNUC__ >= 4
+#if __GNUC__ >= 4 && defined(CONFIG_X86_FAST_FEATURE_TESTS)
 extern void warn_pre_alternatives(void);
 extern bool __static_cpu_has_safe(u16 bit);
 
-- 
2.3.5

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