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

[Brian Gerst]

On Mon, Mar 21, 2016 at 2:54 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> 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.
>
> -Andi

It would be less complicated actually, as normal userspace would just
continue to call arch_prctl() as it does today.  Glibc would implement
arch_prctl() just like it does with gettimeofday() -- with an ifunc
selector [1] that calls the VDSO function if it is available, or the
syscall if not.  No custom assembly needed.

[1] https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/x86/gettimeofday.c;h=36f7c26ffb0e818709d032c605fec8c4bd22a14e;hb=HEAD

--
Brian Gerst