Re: [Qemu-devel] [PATCH v6 11/14] target/arm: PMU: Add instruction and cycle events

[Aaron Lindsay]

On Oct 16 17:04, Richard Henderson wrote:
> On 10/10/18 1:37 PM, Aaron Lindsay wrote:
> > + * Return the underlying cycle count for the PMU cycle counters. If we're 
> > in
> > + * usermode, simply return 0.
> > + */
> > +static uint64_t cycles_get_count(CPUARMState *env)
> > +{
> > +#ifndef CONFIG_USER_ONLY
> > +    return muldiv64(qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL),
> > +                   ARM_CPU_FREQ, NANOSECONDS_PER_SECOND);
> > +#else
> > +    return 0;
> > +#endif
> > +}
> 
> Usually we pass through the host cycle counter.
> See cpu_get_host_ticks().

Why do you prefer cpu_get_host_ticks()? And are you suggesting this for
just user-mode, or both system and user?

PMCCNTR used this same qemu_clock_get_ns() call previous to my patch
(see where this patch replaces that call with one to cycles_get_count()
in pmccntr_op_start()). Of course, we could keep the preexisting PMCCNTR
behavior while making the new cycle counter use cpu_get_host_ticks(),
but having two ways through the same interface which count cycles
differently feels wrong.

-Aaron