Re: [Qemu-devel] [PATCH] target-arm: Use common CPU cycle infrastructure

[Peter Maydell]

On 6 October 2015 at 13:58, Paolo Bonzini <address@hidden> wrote:
>
>
> On 06/10/2015 14:49, Peter Maydell wrote:
>> Yeah, there's no guarantee on userspace access. I think the
>> fastest way to get some kind of count is to use a library
>> function that boils down to gettimeofday(2), which we will
>> do purely in userspace in the kernel VDSO if possible.

Looking closer, clock_gettime() also has a userspace
only fastpath in the VDSO.

> Could we just use CNTVCT_EL0?  Which cores have it?

Not guaranteed to be enabled for userspace access by the kernel,
and even if it is enabled, the kernel folks don't (last time I
checked) consider this userspace ABI -- it's only there for
the benefit of the VDSO. (CNTVCT_EL0 is how the fast clock_gettime
and getimeofday paths are implemented.)

thanks
-- PMM