x86: Add a static_cpu_has_safe variant

We want to use this in early code where alternatives might not have run
yet and for that case we fall back to the dynamic boot_cpu_has.

For that, force a 5-byte jump since the compiler could be generating
differently sized jumps for each label.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1370772454-6106-5-git-send-email-bp@alien8.de
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

2 files changed, 91 insertions, 1 deletions

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 252b28f1176a..47538a61c91b 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -357,6 +357,7 @@ extern const char * const x86_power_flags[32];
 
 #if __GNUC__ >= 4
 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().
@@ -437,11 +438,94 @@ static __always_inline __pure bool __static_cpu_has(u16 bit)
                 __static_cpu_has(bit) :                         \
                 boot_cpu_has(bit)                               \
 )
+
+static __always_inline __pure bool _static_cpu_has_safe(u16 bit)
+{
+#if __GNUC__ > 4 || __GNUC_MINOR__ >= 5
+/*
+ * We need to spell the jumps to the compiler because, depending on the offset,
+ * the replacement jump can be bigger than the original jump, and this we cannot
+ * have. Thus, we force the jump to the widest, 4-byte, signed relative
+ * offset even though the last would often fit in less bytes.
+ */
+                asm goto("1: .byte 0xe9\n .long %l[t_dynamic] - 2f\n"
+                         "2:\n"
+                         ".section .altinstructions,\"a\"\n"
+                         " .long 1b - .\n"              /* src offset */
+                         " .long 3f - .\n"              /* repl offset */
+                         " .word %P1\n"                 /* always replace */
+                         " .byte 2b - 1b\n"             /* src len */
+                         " .byte 4f - 3f\n"             /* repl len */
+                         ".previous\n"
+                         ".section .altinstr_replacement,\"ax\"\n"
+                         "3: .byte 0xe9\n .long %l[t_no] - 2b\n"
+                         "4:\n"
+                         ".previous\n"
+                         ".section .altinstructions,\"a\"\n"
+                         " .long 1b - .\n"              /* src offset */
+                         " .long 0\n"                   /* no replacement */
+                         " .word %P0\n"                 /* feature bit */
+                         " .byte 2b - 1b\n"             /* src len */
+                         " .byte 0\n"                   /* repl len */
+                         ".previous\n"
+                         : : "i" (bit), "i" (X86_FEATURE_ALWAYS)
+                         : : t_dynamic, t_no);
+                return true;
+        t_no:
+                return false;
+        t_dynamic:
+                return __static_cpu_has_safe(bit);
+#else /* GCC_VERSION >= 40500 */
+                u8 flag;
+                /* Open-coded due to __stringify() in ALTERNATIVE() */
+                asm volatile("1: movb $2,%0\n"
+                             "2:\n"
+                             ".section .altinstructions,\"a\"\n"
+                             " .long 1b - .\n"          /* src offset */
+                             " .long 3f - .\n"          /* repl offset */
+                             " .word %P2\n"             /* always replace */
+                             " .byte 2b - 1b\n"         /* source len */
+                             " .byte 4f - 3f\n"         /* replacement len */
+                             ".previous\n"
+                             ".section .discard,\"aw\",@progbits\n"
+                             " .byte 0xff + (4f-3f) - (2b-1b)\n" /* size check */
+                             ".previous\n"
+                             ".section .altinstr_replacement,\"ax\"\n"
+                             "3: movb $0,%0\n"
+                             "4:\n"
+                             ".previous\n"
+                             ".section .altinstructions,\"a\"\n"
+                             " .long 1b - .\n"          /* src offset */
+                             " .long 5f - .\n"          /* repl offset */
+                             " .word %P1\n"             /* feature bit */
+                             " .byte 4b - 3b\n"         /* src len */
+                             " .byte 6f - 5f\n"         /* repl len */
+                             ".previous\n"
+                             ".section .discard,\"aw\",@progbits\n"
+                             " .byte 0xff + (6f-5f) - (4b-3b)\n" /* size check */
+                             ".previous\n"
+                             ".section .altinstr_replacement,\"ax\"\n"
+                             "5: movb $1,%0\n"
+                             "6:\n"
+                             ".previous\n"
+                             : "=qm" (flag)
+                             : "i" (bit), "i" (X86_FEATURE_ALWAYS));
+                return (flag == 2 ? __static_cpu_has_safe(bit) : flag);
+#endif
+}
+
+#define static_cpu_has_safe(bit)                                \
+(                                                               \
+        __builtin_constant_p(boot_cpu_has(bit)) ?               \
+                boot_cpu_has(bit) :                             \
+                _static_cpu_has_safe(bit)                       \
+)
 #else
 /*
  * gcc 3.x is too stupid to do the static test; fall back to dynamic.
  */
-#define static_cpu_has(bit) boot_cpu_has(bit)
+#define static_cpu_has(bit)             boot_cpu_has(bit)
+#define static_cpu_has_safe(bit)        boot_cpu_has(bit)
 #endif
 
 #define cpu_has_bug(c, bit)     cpu_has(c, (bit))
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 59adaa17e220..a4a07c0acb1f 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1372,3 +1372,9 @@ 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);