Re: [PATCH v5 00/24] ARM64 PMU Partitioning

Will Deacon [off-list ref] writes:

On Tue, Dec 09, 2025 at 03:00:59PM -0800, Oliver Upton wrote:

quoted

On Tue, Dec 09, 2025 at 08:50:57PM +0000, Colton Lewis wrote:

quoted

This series creates a new PMU scheme on ARM, a partitioned PMU that
allows reserving a subset of counters for more direct guest access,
significantly reducing overhead. More details, including performance
benchmarks, can be read in the v1 cover letter linked below.

An overview of what this series accomplishes was presented at KVM
Forum 2025. Slides [1] and video [2] are linked below.

The long duration between v4 and v5 is due to time spent on this
project being monopolized preparing this feature for internal
production. As a result, there are too many improvements to fully list
here, but I will cover the notable ones.

quoted

Thanks for reposting. I think there's still quite a bit of ground to
cover on the KVM side of this, but I would definitely appreciate it if
someone with more context on the perf side of things could chime in.

quoted

Will, IIRC you had some thoughts around counter allocation, right?

Right, I was hoping that the host counter reservation could be more
dynamic than a cmdline option. Perf already has support for pinning
events to a CPU, so the concept of some counters being unavailable
shouldn't be too much for the driver to handle. You might just need to
create some fake pinned events so that perf code understands what is
happening.

Thanks Will. I have a few followup questions:

1. Are you suggesting this be done whenever we enter a guest so the host
always has access to the full range in host context? That would be the
most dynamic.

2. How should we handle the possibility a real event already occupies a
counter wanted by the guest? Is there a good way to create our fake
pinned events then force a reschedule so perf moves the real events out
of the way?

3. Is there an existing fake event type that tells perf not to touch
hardware?

4. Can you point to any example code that already does something like
this?