linux-kernel - [PATCH] x86: static_cpu_has_safe: discard dynamic check after init

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Date:	Sat, 16 Jan 2016 14:22:04 -0500
From:	Brian Gerst <brgerst@...il.com>
To:	x86@...nel.org, linux-kernel@...r.kernel.org
Cc:	Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...e.de>
Subject: [PATCH] x86: static_cpu_has_safe: discard dynamic check after init

Move the code to do the dynamic check to the init text section so that it
is discarded after alternatives have run and a static branch has been
chosen.

A new section is defined to avoid warnings with modpost due to references
to init text from main text, which in this case is legitimate.  All such
references are patched out before init mem is discarded.

Signed-off-by: Brian Gerst <brgerst@...il.com>
---
 arch/x86/include/asm/cpufeature.h | 33 ++++++++++++++++++++++-----------
 arch/x86/kernel/cpu/common.c      |  6 ------
 arch/x86/kernel/vmlinux.lds.S     |  6 ++++++
 3 files changed, 28 insertions(+), 17 deletions(-)

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 7ad8c94..2efbd83 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -412,7 +412,6 @@ extern const char * const x86_bug_flags[NBUGINTS*32];
 
 #if __GNUC__ >= 4 && defined(CONFIG_X86_FAST_FEATURE_TESTS)
 extern void warn_pre_alternatives(void);
-extern bool __static_cpu_has_safe(u16 bit);
 
 /*
  * Static testing of CPU features.  Used the same as boot_cpu_has().
@@ -502,10 +501,10 @@ static __always_inline __pure bool __static_cpu_has(u16 bit)
 		boot_cpu_has(bit)				\
 )
 
-static __always_inline __pure bool _static_cpu_has_safe(u16 bit)
+static __always_inline __pure bool _static_cpu_has_safe(u16 bit, __u32 *caps)
 {
 #ifdef CC_HAVE_ASM_GOTO
-		asm_volatile_goto("1: jmp %l[t_dynamic]\n"
+		asm_volatile_goto("1: jmp 6f\n"
 			 "2:\n"
 			 ".skip -(((5f-4f) - (2b-1b)) > 0) * "
 			         "((5f-4f) - (2b-1b)),0x90\n"
@@ -530,17 +529,22 @@ static __always_inline __pure bool _static_cpu_has_safe(u16 bit)
 			 " .byte 0\n"			/* repl len */
 			 " .byte 0\n"			/* pad len */
 			 ".previous\n"
-			 : : "i" (bit), "i" (X86_FEATURE_ALWAYS)
-			 : : t_dynamic, t_no);
+			 ".section .static_cpu_has,\"ax\"\n"
+			 "6: testl %2,%3\n"
+			 "   jnz %l[t_yes]\n"
+			 "   jmp %l[t_no]\n"
+			 ".previous\n"
+			 : : "i" (bit), "i" (X86_FEATURE_ALWAYS),
+			     "i" (1 << (bit & 31)), "m" (caps[bit/32])
+			 : : t_yes, t_no);
+	t_yes:
 		return true;
 	t_no:
 		return false;
-	t_dynamic:
-		return __static_cpu_has_safe(bit);
 #else
 		u8 flag;
 		/* Open-coded due to __stringify() in ALTERNATIVE() */
-		asm volatile("1: movb $2,%0\n"
+		asm volatile("1: jmp 7f\n"
 			     "2:\n"
 			     ".section .altinstructions,\"a\"\n"
 			     " .long 1b - .\n"		/* src offset */
@@ -572,9 +576,15 @@ static __always_inline __pure bool _static_cpu_has_safe(u16 bit)
 			     "5: movb $1,%0\n"
 			     "6:\n"
 			     ".previous\n"
+			     ".section .static_cpu_has,\"ax\"\n"
+			     "7: testl %3,%4\n"
+			     "   setnz %0\n"
+			     "   jmp 2b\n"
+			     ".previous\n"
 			     : "=qm" (flag)
-			     : "i" (bit), "i" (X86_FEATURE_ALWAYS));
-		return (flag == 2 ? __static_cpu_has_safe(bit) : flag);
+			     : "i" (bit), "i" (X86_FEATURE_ALWAYS),
+			       "i" (1 << (bit & 31)), "m" (caps[bit/32]));
+		return (flag != 0);
 #endif /* CC_HAVE_ASM_GOTO */
 }
 
@@ -582,7 +592,8 @@ static __always_inline __pure bool _static_cpu_has_safe(u16 bit)
 (								\
 	__builtin_constant_p(boot_cpu_has(bit)) ?		\
 		boot_cpu_has(bit) :				\
-		_static_cpu_has_safe(bit)			\
+		_static_cpu_has_safe(bit,			\
+			 &boot_cpu_data.x86_capability[0])	\
 )
 #else
 /*
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 37830de..897c65b 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1483,12 +1483,6 @@ void warn_pre_alternatives(void)
 EXPORT_SYMBOL_GPL(warn_pre_alternatives);
 #endif
 
-inline bool __static_cpu_has_safe(u16 bit)
-{
-	return boot_cpu_has(bit);
-}
-EXPORT_SYMBOL_GPL(__static_cpu_has_safe);
-
 static void bsp_resume(void)
 {
 	if (this_cpu->c_bsp_resume)
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 4f19942..4df1467 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -195,6 +195,12 @@ SECTIONS
 	:init
 #endif
 
+	.static_cpu_has : AT(ADDR(.static_cpu_has) - LOAD_OFFSET) {
+		__static_cpu_has_start = .;
+		*(.static_cpu_has)
+		__static_cpu_has_end = .;
+	}
+
 	INIT_DATA_SECTION(16)
 
 	.x86_cpu_dev.init : AT(ADDR(.x86_cpu_dev.init) - LOAD_OFFSET) {
-- 
2.5.0