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

[Andy Lutomirski]

On Wed, Sep 10, 2014 at 9:47 AM, Will Deacon <will.deacon at arm.com> wrote:
> Hi Nathan,
>
> 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.

I have no further bright ideas, unless aarch64 and arm assembly are
miraculously nearly compatible, in which case you could do something
like what x32 does (build 64-bit and then use objcopy to turn the
result into an x32 object).

--Andy