Re: [PATCH v4 1/2] dt-bindings: cpufreq: add virtual cpufreq device

[Saravana Kannan]

Sorry for the delay in response. Was very busy for a while and then
holidays started.

On Fri, Dec 8, 2023 at 12:52 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Thu, 07 Dec 2023 22:44:36 +0000,
> Saravana Kannan <saravanak@xxxxxxxxxx> wrote:
> >
> > On Wed, Nov 15, 2023 at 12:49 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > >
> > > On Sat, 11 Nov 2023 01:49:29 +0000,
> > > David Dai <davidai@xxxxxxxxxx> wrote:
> > > >
> > > > Adding bindings to represent a virtual cpufreq device.
> > > >
> > > > Virtual machines may expose MMIO regions for a virtual cpufreq device
> > > > for guests to read frequency information or to request frequency
> > > > selection. The virtual cpufreq device has an individual controller for
> > > > each frequency domain.
> > >
> > > I would really refrain form having absolute frequencies here. A
> > > virtual machine can be migrated, and there are *zero* guarantees that
> > > the target system has the same clock range as the source.
> > >
> > > This really should be a relative number, much like the capacity. That,
> > > at least, can be migrated across systems.
> >
> > There's nothing in this patch that mandates absolute frequency.
> > In true KVM philosophy, we leave it to the VMM to decide.
>
> This has nothing to do with KVM. It would apply to any execution
> environment, including QEMU in TCG mode.
>
> To quote the original patch:
>
> +    description:
> +      Address and size of region containing frequency controls for each of the
> +      frequency domains. Regions for each frequency domain is placed
> +      contiugously and contain registers for controlling DVFS(Dynamic Frequency
> +      and Voltage) characteristics. The size of the region is proportional to
> +      total number of frequency domains.
>
> What part of that indicates that *relative* frequencies are
> acceptable? The example explicitly uses the opp-v2 binding, which
> clearly is about absolute frequency.

We can update the doc to make that clearer and update the example too.

> To reiterate: absolute frequencies are not the right tool for the job,
> and they should explicitly be described as relative in the spec. Not
> left as a "whatev'" option for the execution environment to interpret.

I think it depends on the use case. If there's no plan to migrate the
VM across different devices, there's no need to do the unnecessary
normalization back and forth.

And if we can translate between pCPU frequency and a normalized
frequency, we can do the same for whatever made up frequencies too. In
fact, we plan to do exactly that in our internal use cases for this.
There's nothing here that prevents the VMM from doing that.

Also, if there are hardware virtualized performance counters (AMU,
CPPC, etc) that are used for frequency normalization, then we have to
use the real frequencies in those devices otherwise the "current
frequency" can be 2 GHz while the max normalized frequency is 1024
KHz. That'll mess up load tracking.

Thanks,
Saravana