x86: Sanity-check static_cpu_has usage

static_cpu_has may be used only after alternatives have run. Before that
it always returns false if constant folding with __builtin_constant_p()
doesn't happen. And you don't want that.

This patch is the result of me debugging an issue where I overzealously
put static_cpu_has in code which executed before alternatives have run
and had to spend some time with scratching head and cursing at the
monitor.

So add a jump to a warning which screams loudly when we use this
function too early. The alternatives patch that check away in
conjunction with patching the rest of the kernel image.

[ hpa: factored this into its own configuration option.  If we want to
  have an overarching option, it should be an option which selects
  other options, not as a group option in the source code. ]

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1370772454-6106-4-git-send-email-bp@alien8.de
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
index c198b7e13e7b..c6acdf752715 100644
--- a/arch/x86/Kconfig.debug
+++ b/arch/x86/Kconfig.debug
@@ -304,4 +304,14 @@ config DEBUG_NMI_SELFTEST
 
           If unsure, say N.
 
+config X86_DEBUG_STATIC_CPU_HAS
+        bool "Debug alternatives"
+        depends on DEBUG_KERNEL
+        ---help---
+          This option causes additional code to be generated which
+          fails if static_cpu_has() is used before alternatives have
+          run.
+
+          If unsure, say N.
+
 endmenu