Re: [Xen-devel] [RFC PATCH v2 10/12] cpufreq: add dom0-cpufreq driver

[Oleksandr Dmytryshyn <oleksandr.dmytryshyn@xxxxxxxxxxxxxxx>]

On Wed, Oct 22, 2014 at 2:00 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Wed, 2014-10-22 at 10:30 +0100, Jan Beulich wrote:
>> >>> On 22.10.14 at 10:43, <oleksandr.dmytryshyn@xxxxxxxxxxxxxxx> wrote:
>> > On Fri, Oct 17, 2014 at 4:03 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>> >>>>> On 16.10.14 at 13:27, <oleksandr.dmytryshyn@xxxxxxxxxxxxxxx> wrote:
>> >>> +static void notify_cpufreq_domains(void)
>> >> Why "domains", not "hwdom"? You don't really want to send this
>> >> to other than the hardware domain I hope.
>> > All domains (not only hwdomain) should receive this interrupt.
>> > In case is hwdomain is Linux kernel it can automaticaly recalculate
>> > jiffies. But other domains should recalculate jiffies too. I'll
>> > implement this mechanism a bit later.
>>
>> This needs more explanation then: If all domains need to do
>> adjustments when the frequency changes, there's something
>> wrong here imo. Such changes are supposed to be visible to
>> the hypervisor only (with, in the case here, assistance - but
>> nothing else - by the hardware domain).
>
> Guests should be using the arch/generic timers as their time source.
> This is required by the hardware to increment at a fixed frequency, so
> guests shouldn't need to be aware of the change to the underlying
> processor clock, at least not for timekeeping/jiffies purposes.
>
> The timers are allowed to increment by larger amounts less frequency in
> lower power modes, i.e. +=4 every 4 ticks. Guests shouldn't need to
> adjust anything for this though IMHO.
There are many functions which are using jiffies. What about them?
All those functions will measure time not correctly if frequency
of the microprocessor will change.

> Ian.
>

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