x86/asm/entry/64: Use a define for an invalid segment selector

... instead of a naked number, for better readability.
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1428054130-25847-1-git-send-email-bp@alien8.deSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/segment.h

@@ -39,6 +39,8 @@
 /* ... GDT has it cleared */
 #define SEGMENT_GDT		0x0
 
+#define GDT_ENTRY_INVALID_SEG	0
+
 #ifdef CONFIG_X86_32
 /*
  * The layout of the per-CPU GDT under Linux:

arch/x86/kernel/cpu/common.c

@@ -1184,7 +1184,7 @@ void syscall_init(void)
 	wrmsrl_safe(MSR_IA32_SYSENTER_EIP, (u64)ia32_sysenter_target);
 #else
 	wrmsrl(MSR_CSTAR, ignore_sysret);
-	wrmsrl_safe(MSR_IA32_SYSENTER_CS, 0ULL);
+	wrmsrl_safe(MSR_IA32_SYSENTER_CS, (u64)GDT_ENTRY_INVALID_SEG);
 	wrmsrl_safe(MSR_IA32_SYSENTER_ESP, 0ULL);
 	wrmsrl_safe(MSR_IA32_SYSENTER_EIP, 0ULL);
 #endif