x86, alternatives: Use 16-bit numbers for cpufeature index

We already have cpufeature indicies above 255, so use a 16-bit number
for the alternatives index.  This consumes a padding field and so
doesn't add any size, but it means that abusing the padding field to
create assembly errors on overflow no longer works.  We can retain the
test simply by redirecting it to the .discard section, however.

[ v3: updated to include open-coded locations ]

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
LKML-Reference: <tip-f88731e3068f9d1392ba71cc9f50f035d26a0d4f@git.kernel.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

2 files changed, 12 insertions, 9 deletions

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 03b6bb5394a0..bc6abb7bc7ee 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -45,10 +45,9 @@
 struct alt_instr {
         u8 *instr;              /* original instruction */
         u8 *replacement;
-        u8  cpuid;              /* cpuid bit set for replacement */
+        u16 cpuid;              /* cpuid bit set for replacement */
         u8  instrlen;           /* length of original instruction */
         u8  replacementlen;     /* length of new instruction, <= instrlen */
-        u8  pad1;
 #ifdef CONFIG_X86_64
         u32 pad2;
 #endif
@@ -86,9 +85,11 @@ static inline int alternatives_text_reserved(void *start, void *end)
       _ASM_ALIGN "\n"                                                   \
       _ASM_PTR "661b\n"                         /* label           */   \
       _ASM_PTR "663f\n"                         /* new instruction */   \
-      "  .byte " __stringify(feature) "\n"      /* feature bit     */   \
+      "  .word " __stringify(feature) "\n"      /* feature bit     */   \
       "  .byte 662b-661b\n"                     /* sourcelen       */   \
       "  .byte 664f-663f\n"                     /* replacementlen  */   \
+      ".previous\n"                                                     \
+      ".section .discard,\"aw\",@progbits\n"                            \
       "  .byte 0xff + (664f-663f) - (662b-661b)\n" /* rlen <= slen */   \
       ".previous\n"                                                     \
       ".section .altinstr_replacement, \"ax\"\n"                        \
diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 468145914389..e8b88967de35 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -291,7 +291,7 @@ extern const char * const x86_power_flags[32];
  * patch the target code for additional performance.
  *
  */
-static __always_inline __pure bool __static_cpu_has(u8 bit)
+static __always_inline __pure bool __static_cpu_has(u16 bit)
 {
 #if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5)
                 asm goto("1: jmp %l[t_no]\n"
@@ -300,11 +300,11 @@ static __always_inline __pure bool __static_cpu_has(u8 bit)
                          _ASM_ALIGN "\n"
                          _ASM_PTR "1b\n"
                          _ASM_PTR "0\n"         /* no replacement */
-                         " .byte %P0\n"         /* feature bit */
+                         " .word %P0\n"         /* feature bit */
                          " .byte 2b - 1b\n"     /* source len */
                          " .byte 0\n"           /* replacement len */
-                         " .byte 0xff + 0 - (2b-1b)\n"  /* padding */
                          ".previous\n"
+                         /* skipping size check since replacement size = 0 */
                          : : "i" (bit) : : t_no);
                 return true;
         t_no:
@@ -318,10 +318,12 @@ static __always_inline __pure bool __static_cpu_has(u8 bit)
                              _ASM_ALIGN "\n"
                              _ASM_PTR "1b\n"
                              _ASM_PTR "3f\n"
-                             " .byte %P1\n"             /* feature bit */
+                             " .word %P1\n"             /* feature bit */
                              " .byte 2b - 1b\n"         /* source len */
                              " .byte 4f - 3f\n"         /* replacement len */
-                             " .byte 0xff + (4f-3f) - (2b-1b)\n" /* padding */
+                             ".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 $1,%0\n"
@@ -337,7 +339,7 @@ static __always_inline __pure bool __static_cpu_has(u8 bit)
 (                                                               \
         __builtin_constant_p(boot_cpu_has(bit)) ?               \
                 boot_cpu_has(bit) :                             \
-        (__builtin_constant_p(bit) && !((bit) & ~0xff)) ?       \
+        __builtin_constant_p(bit) ?                             \
                 __static_cpu_has(bit) :                         \
                 boot_cpu_has(bit)                               \
 )