Re: [PATCH v6 2/2] cpufreq: add virtual-cpufreq driver

[Marc Zyngier <maz@kernel.org>]

On Fri, 28 Jun 2024 13:51:06 +0100,
Sudeep Holla <sudeep.holla@arm.com> wrote:
> 
> On Mon, May 20, 2024 at 09:30:52PM -0700, David Dai wrote:
> > Introduce a virtualized cpufreq driver for guest kernels to improve
> > performance and power of workloads within VMs.
> >
> > This driver does two main things:
> >
> > 1. Sends the frequency of vCPUs as a hint to the host. The host uses the
> > hint to schedule the vCPU threads and decide physical CPU frequency.
> >
> > 2. If a VM does not support a virtualized FIE(like AMUs), it queries the
> > host CPU frequency by reading a MMIO region of a virtual cpufreq device
> > to update the guest's frequency scaling factor periodically. This enables
> > accurate Per-Entity Load Tracking for tasks running in the guest.
> >
> > +
> > +/*
> > + * CPU0..CPUn
> > + * +-------------+-------------------------------+--------+-------+
> > + * | Register    | Description                   | Offset |   Len |
> > + * +-------------+-------------------------------+--------+-------+
> > + * | cur_perf    | read this register to get     |    0x0 |   0x4 |
> > + * |             | the current perf (integer val |        |       |
> > + * |             | representing perf relative to |        |       |
> > + * |             | max performance)              |        |       |
> > + * |             | that vCPU is running at       |        |       |
> > + * +-------------+-------------------------------+--------+-------+
> > + * | set_perf    | write to this register to set |    0x4 |   0x4 |
> > + * |             | perf value of the vCPU        |        |       |
> > + * +-------------+-------------------------------+--------+-------+
> > + * | perftbl_len | number of entries in perf     |    0x8 |   0x4 |
> > + * |             | table. A single entry in the  |        |       |
> > + * |             | perf table denotes no table   |        |       |
> > + * |             | and the entry contains        |        |       |
> > + * |             | the maximum perf value        |        |       |
> > + * |             | that this vCPU supports.      |        |       |
> > + * |             | The guest can request any     |        |       |
> > + * |             | value between 1 and max perf  |        |       |
> > + * |             | when perftbls are not used.   |        |       |
> > + * +---------------------------------------------+--------+-------+
> > + * | perftbl_sel | write to this register to     |    0xc |   0x4 |
> > + * |             | select perf table entry to    |        |       |
> > + * |             | read from                     |        |       |
> > + * +---------------------------------------------+--------+-------+
> > + * | perftbl_rd  | read this register to get     |   0x10 |   0x4 |
> > + * |             | perf value of the selected    |        |       |
> > + * |             | entry based on perftbl_sel    |        |       |
> > + * +---------------------------------------------+--------+-------+
> > + * | perf_domain | performance domain number     |   0x14 |   0x4 |
> > + * |             | that this vCPU belongs to.    |        |       |
> > + * |             | vCPUs sharing the same perf   |        |       |
> > + * |             | domain number are part of the |        |       |
> > + * |             | same performance domain.      |        |       |
> > + * +-------------+-------------------------------+--------+-------+
> > + */
> 
> I think it is good idea to version this table, so that it gives flexibility
> to update the entries. It is a must if we are getting away with DT. I didn't
> give complete information in my previous response where I agreed with Rafael.
> 
> I am not sure how much feasible it is, but can it be queried via KVM IOCTLs
> to VMM. Just a thought, I am exploring how to make this work even on ACPI
> systems. It is simpler if we neednot rely on DT or ACPI.

KVM should not have to know any of this. This is purely between a
contract (and a pretty weak one) between userspace and the guest.

	M.

-- 
Without deviation from the norm, progress is not possible.