Re: [PATCH 4/9] x86: Enumerate kernel FSGS capability in AT_HWCAP2

[Brian Gerst]

On Mon, Mar 21, 2016 at 12:16 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>
> 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.

How about adding a VDSO function instead?  The VDSO can use
alternatives, so it can use the new instructions if supported, or else
use the old syscall.

--
Brian Gerst