[RFC PATCH v3 09/29] KVM: arm64: Hide IMPLEMENTATION DEFINED PMU support for the guest

[Reiji Watanabe]

Hi Eric,

On Thu, Dec 2, 2021 at 2:57 AM Eric Auger <eauger at redhat.com> wrote:
>
> Hi Reiji,
>
> On 11/30/21 6:32 AM, Reiji Watanabe wrote:
> > Hi Eric,
> >
> > On Thu, Nov 25, 2021 at 12:30 PM Eric Auger <eauger at redhat.com> wrote:
> >>
> >> Hi Reiji,
> >>
> >> On 11/17/21 7:43 AM, Reiji Watanabe wrote:
> >>> When ID_AA64DFR0_EL1.PMUVER or ID_DFR0_EL1.PERFMON is 0xf, which
> >>> means IMPLEMENTATION DEFINED PMU supported, KVM unconditionally
> >>> expose the value for the guest as it is.  Since KVM doesn't support
> >>> IMPLEMENTATION DEFINED PMU for the guest, in that case KVM should
> >>> exopse 0x0 (PMU is not implemented) instead.
> >> s/exopse/expose
> >>>
> >>> Change cpuid_feature_cap_perfmon_field() to update the field value
> >>> to 0x0 when it is 0xf.
> >> is it wrong to expose the guest with a Perfmon value of 0xF? Then the
> >> guest should not use it as a PMUv3?
> >
> >> is it wrong to expose the guest with a Perfmon value of 0xF? Then the
> >> guest should not use it as a PMUv3?
> >
> > For the value 0xf in ID_AA64DFR0_EL1.PMUVER and ID_DFR0_EL1.PERFMON,
> > Arm ARM says:
> >   "IMPLEMENTATION DEFINED form of performance monitors supported,
> >    PMUv3 not supported."
> >
> > Since the PMU that KVM supports for guests is PMUv3, 0xf shouldn't
> > be exposed to guests (And this patch series doesn't allow userspace
> > to set the fields to 0xf for guests).
> What I don't get is why this isn't detected before (in kvm_reset_vcpu).
> if the VCPU was initialized with KVM_ARM_VCPU_PMU_V3 can we honor this
> init request if the host pmu is implementation defined?

KVM_ARM_VCPU_INIT with KVM_ARM_VCPU_PMU_V3 will fail in
kvm_reset_vcpu() if the host PMU is implementation defined.

The AA64DFR0 and DFR0 registers for a vCPU without KVM_ARM_VCPU_PMU_V3
indicates IMPLEMENTATION DEFINED PMU support, which is not correct.

Thanks,
Reiji