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>

1 files changed, 8 insertions, 6 deletions

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)                               \
 )