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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 <ak@linux.intel.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Link: https://lkml.kernel.org/r/1557309753-24073-18-git-send-email-chang.seok.bae@intel.com
This commit is contained in:
Andi Kleen 2019-05-08 03:02:32 -07:00 committed by Thomas Gleixner
parent 2032f1f96e
commit f987c955c7
2 changed files with 6 additions and 1 deletions
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3
arch/x86/include/uapi/asm/hwcap2.h
View File

@ -5,4 +5,7 @@
/* MONITOR/MWAIT enabled in Ring 3 */ /* MONITOR/MWAIT enabled in Ring 3 */
#define HWCAP2_RING3MWAIT (1 << 0) #define HWCAP2_RING3MWAIT (1 << 0)
/* Kernel allows FSGSBASE instructions available in Ring 3 */
#define HWCAP2_FSGSBASE BIT(1)
#endif #endif

4
arch/x86/kernel/cpu/common.c
View File

@ -1387,8 +1387,10 @@ static void identify_cpu(struct cpuinfo_x86 *c)
setup_umip(c); setup_umip(c);
/* Enable FSGSBASE instructions if available. */ /* 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); cr4_set_bits(X86_CR4_FSGSBASE);
elf_hwcap2 |= HWCAP2_FSGSBASE;
}
/* /*
* The vendor-specific functions might have changed features. * The vendor-specific functions might have changed features.