[PATCH V2 14/15] x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2

["Chang S. Bae" <chang.seok.bae@intel.com>]

From: Andi Kleen <ak@linux.intel.com>

The kernel needs to explicitly enable FSGSBASE. So, the application
needs to know if it can safely use these instructions. Just looking
at the CPUID bit is not enough because it may be running in a kernel
that does not enable the instructions.

One way for the application would be to just try and catch the SIGILL.
But that is difficult to do in libraries which may not want
to overwrite the signal handlers of the main application.

So we need to provide a way for the application to discover the kernel
capability.

I used AT_HWCAP2 in the ELF aux vector which is already used by
PPC for similar things. We define a new Linux defined bitmap
returned in AT_HWCAP.  Next to MONITOR/MWAIT, bit 1 is reserved for
FSGSBASE capability checks.

The application can then access it manually or using
the getauxval() function in newer glibc.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
[chang: Rebase and edit the patch note accordingly]
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/include/uapi/asm/hwcap2.h | 3 +++
 arch/x86/kernel/cpu/common.c       | 4 +++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/uapi/asm/hwcap2.h b/arch/x86/include/uapi/asm/hwcap2.h
index 6ebaae9..c5ce54e 100644
--- a/arch/x86/include/uapi/asm/hwcap2.h
+++ b/arch/x86/include/uapi/asm/hwcap2.h
@@ -5,4 +5,7 @@
 /* MONITOR/MWAIT enabled in Ring 3 */
 #define HWCAP2_RING3MWAIT		(1 << 0)
 
+/* Kernel allows FSGSBASE instructions available in Ring 3 */
+#define HWCAP2_FSGSBASE			BIT(1)
+
 #endif
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 0339bb3..5e6b46c 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1303,8 +1303,10 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	setup_umip(c);
 
 	/* Enable FSGSBASE instructions if available. */
-	if (cpu_has(c, X86_FEATURE_FSGSBASE))
+	if (cpu_has(c, X86_FEATURE_FSGSBASE)) {
 		cr4_set_bits(X86_CR4_FSGSBASE);
+		elf_hwcap2 |= HWCAP2_FSGSBASE;
+	}
 
 	/*
 	 * The vendor-specific functions might have changed features.
-- 
2.7.4