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

From: Andi Kleen
Date: Mon Mar 21 2016 - 15:32:03 EST

On Mon, Mar 21, 2016 at 02:49:44PM -0400, Brian Gerst wrote:
> 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.

What would be the point of that?

It would be a lot more complicated, and I don't see any advantages
over the aux vector. vdso also requires custom assembler
stubs in the C library.