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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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.

Ian.


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