Re: [Qemu-devel] [RFC PATCH V4 0/6] icount: Implement delay algorithm between guest and host clocks

[Paolo Bonzini]

Il 22/07/2014 16:02, Sebastian Tanase ha scritto:
> Yes, QEMU_CLOCK_VIRTUAL counts up from qemu_clock_get_ns(QEMU_CLOCK_REALTIME)
> on ARM (I have only tested with the versatilepb and vexpress boards).

That's a bug to fix indeed, then---it should count up from 0 without
icount, and icount shouldn't affect this.  Thanks for investigating it.

> Supposing the patch that changes vm_clock_warp_start from 0 to -1 is accepted,

... which shouldn't be a problem,... :)

> I could use the information in timers_state.cpu_clock_offset instead of 
> recalculating
> the offset. Besides, given that I only need this particular field from the 
> whole 
> structure, I think I don't have to make timers_state public; I could add a 
> function 
> in cpus.c, for example:
> 
>     int64_t cpu_get_clock_offset(void)
>     {
>         int64_t ti;
>         unsigned start;
> 
>         do {
>             start = seqlock_read_begin(&timers_state.vm_clock_seqlock);
>             ti = -timers_state.cpu_clock_offset;
>         } while (seqlock_read_retry(&timers_state.vm_clock_seqlock, start));
> 
>         return ti;
>     }
> 
> that will return the cpu_clock_offset field.

Indeed what I was proposing is a bit more sloppy.  If you do that, you
have to make the function a bit more general:

    ti = timers_state.cpu_clock_offset;
    if (!timers_state.cpu_ticks_enabled) {
        ti -= get_clock();
    }
    ...

    return -ti;

even though in cpus.c you'll only be using it when cpu_ticks_enabled is
true.  See cpu_enable_ticks() and cpu_disable_ticks().

Paolo