From: Andi Kleen <a...@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. [ chang: Rebase and edit the patch note accordingly. ] Signed-off-by: Andi Kleen <a...@linux.intel.com> Signed-off-by: Chang S. Bae <chang.seok....@intel.com> Cc: Andy Lutomirski <l...@kernel.org> Cc: H. Peter Anvin <h...@zytor.com> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Ingo Molnar <mi...@kernel.org> Cc: Dave Hansen <dave.han...@linux.intel.com> --- 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 6ebaae90e207..c5ce54e749f6 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 f20edc754532..6964dd24082d 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -1369,8 +1369,10 @@ 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. -- 2.19.1