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

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi,

On Sat, Dec 04, 2021 at 09:39:59AM -0800, Reiji Watanabe wrote:
> 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.

The code was recently changed (in v5.15 I think), I think Eric is looking
at an older version.

> 
> 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 recently started looking into that too. If there's only one PMU, then the
guest won't see the value IMP DEF for PMUVer (userspace cannot set the PMU
feature because !kvm_arm_support_pmu_v3()).

On heterogeneous systems with multiple PMUs, it gets complicated. I don't
have any such hardware, but what I think will happen is that KVM will
enable the static branch if there is at least one PMU with
PMUVer != IMP_DEF, even if there are other PMUs with PMUVer = IMP_DEF. But
read_sanitised_ftr_reg() will always return 0 for the
PMUVer field because the field is defined as FTR_EXACT with a safe value of
0 in cpufeature.c. So the guest ends up seeing PMUVer = 0.

I'm not sure if this is the case because I'm not familiar with the cpu
features code, but I planning to investigate further.

Thanks,
Alex

> (I'm not sure how KVM's PMUV3 support code is implemented in the
> code that you are looking at though)
> 
> Thanks,
> Reiji

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