[PATCH 6/6] aarch64: Add hp-timing.h

[Peter Maydell]

On 3 September 2015 at 14:17, Marcus Shawcroft
<marcus.shawcroft@gmail.com> wrote:
> There are two ways forward:
>
> 1) Kernel folk agree to expose cntvct_el0 for the benefit of general user space.
>
> 2) We add a HWCAP bit to indicate if cntvct_el0 is exposed.
>
> Taking the proposed patch as it stands in the absence of a commitment
> from the kernel doesn't look sensible to me.  Since the loader uses hp
> timers in normal operation we will end up with a dynamic loader that
> faults if a future kernel disables visibility of cntvct_el0.

You need to have the dynamic loader check whether the counter
is available somehow anyway, otherwise it won't work on:
 * older kernels that don't export the counter to userspace
 * QEMU usermode emulation
 * valgrind
 * possibly other similar JIT tools

Obviously valgrind/QEMU/etc can be improved to add the counter
support, but there are still going to be old versions out in the
wild.

thanks
-- PMM