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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2014-10-22 at 16:24 +0300, Oleksandr Dmytryshyn wrote:
> 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.

Jiffies is based on the timer tick, isn't it? Therefore it is ultimately
based on the arch timer.

Ian.



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