aarch64 lacks HP_TIMING support in glibc

[Adhemerval Zanella]

On 22/06/2021 15:38, Andrew Pinski via Libc-alpha wrote:
> On Tue, Jun 22, 2021 at 11:30 AM Rafael Eduardo Ruviaro Christ via
> Libc-alpha <libc-alpha@sourceware.org> wrote:
>>
>> Hello,
>>
>> I noticed that aarch64 lacks HP_TIMING support in glibc. Although this architecture has generic timers, glibc is still using the generic hp-timing.h for aarch64.
>> I see HP_TIMING support for other architectures. I'd like to know why there is no HP_TIMING support for aarch64?
>> Also, if there's planning to support it for aarch64?
> 
> See https://sourceware.org/legacy-ml/libc-alpha/2015-06/msg00736.html
> and the previous emails in the thread.
> 


Do you have an outcome from kernel regarding it? It seems that the vDSO
seems to use cntvct_el0 regardless, even if it traps for emulation (not
sure if kernel rewrites the vDSO depending of underlying kernel support).

It should be possible support hp-timing.h on aarch64 to use cntvct_el0
if the minimum supported kernel also supports it (3.2).  Afaik the
hp-timing will be used only for LD_DEBUG=statistics, so default usage
would not be penalized.