Re: [PATCH v4] arm/kvm: Enable support for KVM_ARM_VCPU_PMU_V3_FILTER

[Sebastian Ott]

On Fri, 15 Dec 2023, Sebastian Ott wrote:
> On Thu, 14 Dec 2023, Eric Auger wrote:
> >  On 12/7/23 11:36, Shaoqin Huang wrote:
> > >  +    if (kvm_vcpu_ioctl(cs, KVM_HAS_DEVICE_ATTR, &attr)) {
> > >  +        warn_report("The kernel doesn't support the PMU Event
> > >  Filter!\n");
> > >  +        return;
> > >  +    }
> > >  +
> > >  +    /* The filter only needs to be initialized for 1 vcpu. */
> >  Are you sure? This is a per vcpu device ctrl. Where is it written in the
> >  doc that this shall not be called for each vcpu
>
> The interface is per vcpu but the filters are actually managed per vm
> (kvm->arch.pmu_filter). From (kernel) commit 6ee7fca2a ("KVM: arm64: Add 
> KVM_ARM_VCPU_PMU_V3_SET_PMU attribute"):
>  To ensure that KVM doesn't expose an asymmetric system to the guest, the
>  PMU set for one VCPU will be used by all other VCPUs. Once a VCPU has run,
>  the PMU cannot be changed in order to avoid changing the list of available
>  events for a VCPU, or to change the semantics of existing events.

I've managed to quote the wrong commit. It's that one:
d7eec2360e3 ("KVM: arm64: Add PMU event filtering infrastructure")
 Note that although the ioctl is per-vcpu, the map of allowed events is
 global to the VM (it can be setup from any vcpu until the vcpu PMU is
 initialized).

Sebastian