Messages in this thread | | | From | Andy Lutomirski <> | Date | Fri, 10 Apr 2015 12:15:47 -0700 | Subject | Re: [PATCH 6/8] x86: Enumerate kernel FSGS capability in AT_HWCAP2 |
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On Fri, Apr 10, 2015 at 8:50 AM, Andi Kleen <andi@firstfloor.org> wrote: > From: Andi Kleen <ak@linux.intel.com> > > The kernel needs to explicitely enable RD/WRFSBASE to handle context > switch correctly. So the application needs to know if it can safely use > these instruction. 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. Currently it has only one bit set, > for kernel is FSGSBASE capable. > > The application can then access it manually or using > the getauxval() function in newer glibc. > > Signed-off-by: Andi Kleen <ak@linux.intel.com> > --- > arch/x86/include/asm/elf.h | 7 +++++++ > arch/x86/include/uapi/asm/hwcap.h | 7 +++++++ > arch/x86/kernel/cpu/common.c | 7 ++++++- > 3 files changed, 20 insertions(+), 1 deletion(-) > create mode 100644 arch/x86/include/uapi/asm/hwcap.h > > diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h > index ca3347a..47dac31 100644 > --- a/arch/x86/include/asm/elf.h > +++ b/arch/x86/include/asm/elf.h > @@ -257,6 +257,13 @@ extern int force_personality32; > > #define ELF_HWCAP (boot_cpu_data.x86_capability[0]) > > +extern unsigned kernel_enabled_features; > + > +/* HWCAP2 supplies kernel enabled CPU feature, so that the application > + can know that it can safely use them. The bits are defined in > + uapi/asm/hwcap.h. */ > +#define ELF_HWCAP2 kernel_enabled_features;
Could we name this something more like elf_hwcap2 instead of kernel_enabled_features?
--Andy
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