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

[Reiji Watanabe <reijiw@google.com>]

Hi Eric,

On Sat, Dec 4, 2021 at 6:14 AM Eric Auger <eauger@redhat.com> wrote:
>
> Hi Reiji,
>
> On 12/4/21 2:04 AM, Reiji Watanabe wrote:
> > Hi Eric,
> >
> > On Thu, Dec 2, 2021 at 2:57 AM Eric Auger <eauger@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@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.
>
> OK. This was not obvsious to me.
>
>                 if (kvm_vcpu_has_pmu(vcpu) && !kvm_arm_support_pmu_v3()) {
>                         ret = -EINVAL;
>                         goto out;
>                 }
>
> kvm_perf_init
> +       if (perf_num_counters() > 0)
> +               static_branch_enable(&kvm_arm_pmu_available);
>
> But I believe you ;-), sorry for the noise

Thank you for the review !

I didn't find the code above in v5.16-rc3, which is the base code of
this series.  So, I'm not sure where the code came from (any kvmarm
repository branch ??).

What I see in v5.16-rc3 is:
----
int kvm_perf_init(void)
{
        return perf_register_guest_info_callbacks(&kvm_guest_cbs);
}

void kvm_host_pmu_init(struct arm_pmu *pmu)
{
        if (pmu->pmuver != 0 && pmu->pmuver != ID_AA64DFR0_PMUVER_IMP_DEF &&
            !kvm_arm_support_pmu_v3() && !is_protected_kvm_enabled())
                static_branch_enable(&kvm_arm_pmu_available);
}
----

And I don't find any other code that enables kvm_arm_pmu_available.

Looking at the KVM's PMUV3 support code for guests in v5.16-rc3,
if KVM allows userspace to configure KVM_ARM_VCPU_PMU_V3 even with
ID_AA64DFR0_PMUVER_IMP_DEF on the host (, which I don't think it does),
I think we should fix that to not allow that.
(I'm not sure how KVM's PMUV3 support code is implemented in the
code that you are looking at though)

Thanks,
Reiji