Re: [PATCH v5 08/12] KVM: arm64: PMU: Allow userspace to limit PMCR_EL0.N for the guest

Raghavendra Rao Ananta
2023-08-21T23:28:48Z

Hi Shaoqin,

On Mon, Aug 21, 2023 at 5:12 AM Shaoqin Huang <shahuang@redhat.com> wrote:
>
> Hi Raghavendra,
>
> On 8/17/23 08:30, Raghavendra Rao Ananta wrote:
> > From: Reiji Watanabe <reijiw@google.com>
> >
> > KVM does not yet support userspace modifying PMCR_EL0.N (With
> > the previous patch, KVM ignores what is written by upserspace).
> > Add support userspace limiting PMCR_EL0.N.
> >
> > Disallow userspace to set PMCR_EL0.N to a value that is greater
> > than the host value (KVM_SET_ONE_REG will fail), as KVM doesn't
> > support more event counters than the host HW implements.
> > Although this is an ABI change, this change only affects
> > userspace setting PMCR_EL0.N to a larger value than the host.
> > As accesses to unadvertised event counters indices is CONSTRAINED
> > UNPREDICTABLE behavior, and PMCR_EL0.N was reset to the host value
> > on every vCPU reset before this series, I can't think of any
> > use case where a user space would do that.
> >
> > Also, ignore writes to read-only bits that are cleared on vCPU reset,
> > and RES{0,1} bits (including writable bits that KVM doesn't support
> > yet), as those bits shouldn't be modified (at least with
> > the current KVM).
> >
> > Signed-off-by: Reiji Watanabe <reijiw@google.com>
> > Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
> > ---
> > arch/arm64/include/asm/kvm_host.h | 3 ++
> > arch/arm64/kvm/pmu-emul.c | 1 +
> > arch/arm64/kvm/sys_regs.c | 49 +++++++++++++++++++++++++++++--
> > 3 files changed, 51 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 0f2dbbe8f6a7e..c15ec365283d1 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -259,6 +259,9 @@ struct kvm_arch {
> > /* PMCR_EL0.N value for the guest */
> > u8 pmcr_n;
> >
> > + /* Limit value of PMCR_EL0.N for the guest */
> > + u8 pmcr_n_limit;
> > +
> > /* Hypercall features firmware registers' descriptor */
> > struct kvm_smccc_features smccc_feat;
> > struct maple_tree smccc_filter;
> > diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> > index ce7de6bbdc967..39ad56a71ad20 100644
> > --- a/arch/arm64/kvm/pmu-emul.c
> > +++ b/arch/arm64/kvm/pmu-emul.c
> > @@ -896,6 +896,7 @@ int kvm_arm_set_vm_pmu(struct kvm *kvm, struct arm_pmu *arm_pmu)
> > * while the latter does not.
> > */
> > kvm->arch.pmcr_n = arm_pmu->num_events - 1;
> > + kvm->arch.pmcr_n_limit = arm_pmu->num_events - 1;
> >
> > return 0;
> > }
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 2075901356c5b..c01d62afa7db4 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -1086,6 +1086,51 @@ static int get_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
> > return 0;
> > }
> >
> > +static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
> > + u64 val)
> > +{
> > + struct kvm *kvm = vcpu->kvm;
> > + u64 new_n, mutable_mask;
> > + int ret = 0;
> > +
> > + new_n = FIELD_GET(ARMV8_PMU_PMCR_N, val);
> > +
> > + mutex_lock(&kvm->arch.config_lock);
> > + if (unlikely(new_n != kvm->arch.pmcr_n)) {
> > + /*
> > + * The vCPU can't have more counters than the PMU
> > + * hardware implements.
> > + */
> > + if (new_n <= kvm->arch.pmcr_n_limit)
> > + kvm->arch.pmcr_n = new_n;
> > + else
> > + ret = -EINVAL;
> > + }
>
> Since we have set the default value of pmcr_n, if we want to set a new
> pmcr_n, shouldn't it be a different value?
>
> So how about change the checking to:
>
> if (likely(new_n <= kvm->arch.pmcr_n_limit)
> kvm->arch.pmcr_n = new_n;
> else
> ret = -EINVAL;
>
> what do you think?
>
Sorry, I guess I didn't fully understand your suggestion. Are you
saying that it's 'likely' that userspace would configure the correct
value?

> > + mutex_unlock(&kvm->arch.config_lock);
> > + if (ret)
> > + return ret;
> > +
> > + /*
> > + * Ignore writes to RES0 bits, read only bits that are cleared on
> > + * vCPU reset, and writable bits that KVM doesn't support yet.
> > + * (i.e. only PMCR.N and bits [7:0] are mutable from userspace)
> > + * The LP bit is RES0 when FEAT_PMUv3p5 is not supported on the vCPU.
> > + * But, we leave the bit as it is here, as the vCPU's PMUver might
> > + * be changed later (NOTE: the bit will be cleared on first vCPU run
> > + * if necessary).
> > + */
> > + mutable_mask = (ARMV8_PMU_PMCR_MASK | ARMV8_PMU_PMCR_N);
> > + val &= mutable_mask;
> > + val |= (__vcpu_sys_reg(vcpu, r->reg) & ~mutable_mask);
> > +
> > + /* The LC bit is RES1 when AArch32 is not supported */
> > + if (!kvm_supports_32bit_el0())
> > + val |= ARMV8_PMU_PMCR_LC;
> > +
> > + __vcpu_sys_reg(vcpu, r->reg) = val;
> > + return 0;
> > +}
> > +
> > /* Silly macro to expand the DBG{BCR,BVR,WVR,WCR}n_EL1 registers in one go */
> > #define DBG_BCR_BVR_WCR_WVR_EL1(n) \
> > { SYS_DESC(SYS_DBGBVRn_EL1(n)), \
> > @@ -2147,8 +2192,8 @@ static const struct sys_reg_desc sys_reg_descs[] = {
> > { SYS_DESC(SYS_CTR_EL0), access_ctr },
> > { SYS_DESC(SYS_SVCR), undef_access },
> >
> > - { PMU_SYS_REG(PMCR_EL0), .access = access_pmcr,
> > - .reset = reset_pmcr, .reg = PMCR_EL0, .get_user = get_pmcr },
> > + { PMU_SYS_REG(PMCR_EL0), .access = access_pmcr, .reset = reset_pmcr,
> > + .reg = PMCR_EL0, .get_user = get_pmcr, .set_user = set_pmcr },
>
> A little confusing, since the PMU_SYS_REG() defines the default
> visibility which is pmu_visibility can return REG_HIDDEN, the set_user
> to pmcr will be blocked, how can it being set?
>
> Maybe I lose some details.
>
pmu_visibility() returns REG_HIDDEN only if userspace has not added
support for PMUv3 via KVM_ARM_PREFERRED_TARGET ioctl. Else, it should
return 0, and give access.

Thank you.
Raghavendra

> Thanks,
> Shaoqin
>
> > { PMU_SYS_REG(PMCNTENSET_EL0),
> > .access = access_pmcnten, .reg = PMCNTENSET_EL0 },
> > { PMU_SYS_REG(PMCNTENCLR_EL0),
>
> --
> Shaoqin
>