Re: [PATCH 1/2] KVM: arm64: Fix MDCR_EL2.HPMN reset value

[Oliver Upton <oliver.upton@linux.dev>]

Hey,

On Mon, Feb 17, 2025 at 11:24:11AM +0000, Marc Zyngier wrote:
> The MDCR_EL2 documentation indicates that the HPMN field has
> the following behaviour:
> 
> "On a Warm reset, this field resets to the expression NUM_PMU_COUNTERS."
> 
> However, it appears we reset it to zero, which is not very useful.
> 
> Add a reset helper for MDCR_EL2, and handle the case where userspace
> changes the target PMU, which may force us to change HPMN again.
> 
> Reported-by: Joey Gouly <joey.gouly@arm.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>

The existing ABI expectations are that writes to PMCR_EL0.N constrain
the number of counters, so that should have a similar effect on
MDCR_EL2.HPMN.

At the same time, I get the feeling that we should throw out this whole
behavior of writing N to change the shape of the PMU, because it
complete breaks down for NV. PMCR_EL0.N is another one of those fields
that change behavior based on EL and isn't a global source of truth on
the shape of the PMU.

What do you think about adding a new vCPU attribute for selecting the
number of counters for a VM? We can allow non-nested VMs to use the
'old' method of writing PMCR_EL0.N and force nested VMs to use the
attribute.

We can then enforce ordering on the attribute and prevent it from being
used after vCPU reset.

Thanks,
Oliver