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

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

On Wed, Feb 19, 2025 at 02:03:49PM +0000, Marc Zyngier wrote:
> On Mon, 17 Feb 2025 18:53:50 +0000, Oliver Upton <oliver.upton@linux.dev> wrote:
> > 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.
> 
> VCPU attribute? or PMU attribute? I'm really not keen on the former,
> but the latter is probably workable, as it is VM-wide, similar to the
> way we keep track of pmcr_n.

Well the _existing_ PMU attributes are actually vCPU attributes. I do
agree that accessing them as a VM attribute makes more sense, but that's
the UAPI we already have...

> > We can then enforce ordering on the attribute and prevent it from being
> > used after vCPU reset.
> 
> How would that work? Do you really want to mandate the PMU selection
> (with its counter capping) to strictly occur between vcpu creation and
> init?
> 
> This would, for example, break kvmtool which has these two operations
> back-to-back, and sneaking new device-specific actions in the middle
> is a bit unpalatable (there is a split between VM-wide and per-vcpu
> actions).
> 
> Any idea?

If we want to do this the 'right' way, we should provide VM attributes
for selecting the PMU implementation / configuring the event filter
to complement an attribute for setting the number of event counters.

I don't want to have a mix-and-match approach where vPMU attributes are
scattered between the vCPU and the VM since it requires a similar amount
of gymnastics in userspace to set crap up.

Thanks,
Oliver