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

[Catalin Marinas]

On Wed, Sep 09, 2015 at 05:33:23PM -0700, Richard Henderson wrote:
> On 09/09/2015 04:14 AM, Catalin Marinas wrote:
> >What are the glibc expectations here for HP_TIMING? Is it fine if it
> >always returns 0? Does its frequency need to be known? If yes, how do we
> >expose such frequency to user space?
> 
> Yes it's fine if it returns 0.  You just won't get useful timing info when
> using LD_DEBUG=statistics.
> 
> No, we don't require a known frequency.  Even on x86 we simply print
> "cycles" rather than a scaled time.
> 
> I think the only real requirement is not crashing due to sigill while
> collecting the statistics. ;-)
> 
> >IIUC here it's a simple HP_TIMING_NOW macro which translates to an mrs.
> 
> I did that simply because it worked for me.  You'll notice the significant
> push-back that patch elicited.  ;-)
> 
> >Do you plan to add further checks for HWCAP here?
> 
> If required.  It ought to be easy.
> 
> >Is exposing a reader function via VDSO be feasible here? Or it's too
> >early for the dynamic loader?
> 
> Good question... it's on the edge, right after self-relocation of ld.so, but
> before proper relocation of libc and re-relocation of ld.so (e.g. to use
> libc symbols like malloc instead of the bootstrapping stubs).
> 
> Finding the VDSO is certainly easy, since its pointer is in auxv, and we
> will have stored that.  But how much effort it is to look up a symbol by
> name for that at that point in the bootstrap, I don't know.
> 
> Would the kernel be likely to provide an implementation that wasn't simply
> the mrs insn?
> 
> If not, I don't know that's any better than the HWCAP bit.  Either way we
> need a test-and-branch.  Whether that's checking for a null pointer because
> the kernel didn't provide the vdso entry point, or checking hwcap for the
> bit, it's just about the same.

Let's start with a HWCAP bit. But someone needs to write a patch and
post it on linux-arm-kernel ;) (being an ABI request, it needs raising
it on the kernel list).

-- 
Catalin