From f987c955c74501c9295a81372c7d363cbe07c8a6 Mon Sep 17 00:00:00 2001 From: Andi Kleen Date: Wed, 8 May 2019 03:02:32 -0700 Subject: [PATCH] x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2 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. Enumerate the enabled FSGSBASE capability in bit 1 of AT_HWCAP2 in the ELF aux vector. AT_HWCAP2 is already used by PPC for similar purposes. The application can access it open coded or by using the getauxval() function in newer versions of glibc. [ tglx: Massaged changelog ] Signed-off-by: Andi Kleen Signed-off-by: Chang S. Bae Signed-off-by: Thomas Gleixner Cc: Andy Lutomirski Cc: Ravi Shankar Cc: H. Peter Anvin Link: https://lkml.kernel.org/r/1557309753-24073-18-git-send-email-chang.seok.bae@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 1305f16b6105..637c9117d5ae 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -1387,8 +1387,10 @@ 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. -- 2.30.2