[PATCH v9 4/6] ARM: add vdso user-space code

[Nathan Lynch]

On 09/10/2014 12:10 PM, Will Deacon wrote:
> On Wed, Sep 10, 2014 at 05:52:39PM +0100, Andy Lutomirski wrote:
>> On Wed, Sep 10, 2014 at 9:47 AM, Will Deacon <will.deacon at arm.com> wrote:
>>> On Fri, Aug 22, 2014 at 10:52:29PM +0100, Nathan Lynch wrote:
>>>> Place vdso-related user-space code in arch/arm/kernel/vdso/.
>>>>
>>>> It is almost completely written in C with some assembly helpers to
>>>> load the data page address, sample the counter, and fall back to
>>>> system calls when necessary.
>>>
>>> I'm still a bit puzzled as to how we can implement a compat version of this
>>> for a 32-bit userspace running under a 64-bit kernel. Maybe the answer is
>>> that we don't care enough (programs will still work fine without it), but if
>>> we did want to then we're going to need to build the kernel with two
>>> toolchains and it gets really horrible.
>>>
>>> Do you have any ideas?
>>
>> Convince the gcc and binutils people to add a-m32 option to aarch64?
>> That's how x86_64 pulls this off :)  Or you could require a
>> cross-compiler to be available to enable this particular feature.
> 
> The compilers have two separate backends, so I think I'll know what they
> say. I guess it's either overhauling kbuild to support two cross compilers,
> or have some shell script to accept an option we make up.

arch/powerpc has addressed this with a CROSS32_COMPILE= variable for
producing 32-bit outputs (compat vdso, bootwrapper) during a 64-bit
kernel build.  I think that was used before powerpc toolchains commonly
honored -m32.