Re: [Qemu-devel] [PATCH] timer.h: Provide monotonic time for ARM guests

[Paolo Bonzini <pbonzini@redhat.com>]

On 17/04/2017 20:55, Pranith Kumar wrote:
>>> +/* ARM does not have a user-space readble cycle counter available.
>>> + * This is a compromise to get monotonically increasing time.
>>> + */
>>> +static inline int64_t cpu_get_host_ticks(void)
>>> +{
>>> +    return get_clock();
>>> +}
>> This doesn't look like it should be ARM-specific. Is it
>> better than the current default implementation? If so,
>> why not make this the default implementation?
>
> I think we can do that...

Yes, it is always better for emulation accuracy.  It may be much slower,
depending on your OS (especially if get_clock requires a
user->kernel->user transition), but the current code is quite broken.

Paolo

>>> +
>>>  #else
>>>  /* The host CPU doesn't have an easily accessible cycle counter.
>>>     Just return a monotonically increasing value.  This will be
>> The comment here says that our default is already a monotonically
>> increasing implementation -- is it wrong, or is there some other
>> advantage of your version?
> Comment #6 in the bug report by Laszlo Ersek explains why.
> Incrementing by 1 for approximately 55 ms is what is causing the
> divide by zero in grub.