[PATCH v2 0/3] arm_arch_timer: VDSO preparation, code consolidation

On 09/22/2014 05:30 PM, Russell King - ARM Linux wrote:

On Mon, Sep 22, 2014 at 04:39:19PM +0100, Will Deacon wrote:

quoted

On Thu, Sep 18, 2014 at 03:59:32PM +0100, Nathan Lynch wrote:

quoted

This series contains the necessary changes to allow architected timer
access from user-space on 32-bit ARM.  This allows the VDSO to support
high resolution timestamps for clock_gettime and gettimeofday.  This
also merges substantially similar code from arm and arm64 into the
core arm_arch_timer driver.

The functional changes are:
- When available, CNTVCT is made readable by user space on arm, as it
  is on arm64.
- The clocksource name becomes "arch_mem_counter" if CP15 access to
  the counter is not available.

These changes have been carried as part of the ARM VDSO patch set over
the last several months, but I am splitting them out here as I assume
they should go through the clocksource maintainers.

For the series:

  Acked-by: Will Deacon [off-list ref]

I'm not sure which tree the arch-timer stuff usually goes through, but
the arm/arm64 bits look fine so I'm happy for them to merged together.

I raised a while back with Will whether there's much point to having
this on ARM.  While it's useful for virtualisation, the majority of
32-bit ARM doesn't run virtualised.  So there's little point in having
the VDSO on the majority of platforms - it will just add additional
unnecessary cycles slowing down the system calls that the VDSO is
designed to try to speed up.

Hmm, this patch set is merely exposing the hardware counter when it is
present for the VDSO's use; I take it you have no objection to that?

While the 32-bit ARM VDSO I've posted (in a different thread) exploits a
facility that is required by the virtualization option in the
architecture, its utility is not limited to guest operating systems.

So, my view is that this VDSO will only be of very limited use for
32-bit ARM, and should not be exposed to userspace unless there is
a reason for it to be exposed (iow, the hardware necessary to support
it is present.)

My thinking is that it should prove useful in a growing subset of v7
CPUs.  It is useful today on Cortex-A15 and -A7, and I believe -A12 and
-A17 implement the generic timer facility as well.

Now if you're saying that we shouldn't slow down gettimeofday on systems
which lack a hardware counter that can be safely exposed to userspace, I
can work with that.