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

[Raghavendra Rao Ananta]

On Thu, Aug 24, 2023 at 1:50 AM Shaoqin Huang <shahuang@xxxxxxxxxx> wrote:
>
>
>
> On 8/24/23 00:06, Raghavendra Rao Ananta wrote:
> > On Tue, Aug 22, 2023 at 3:06 AM Shaoqin Huang <shahuang@xxxxxxxxxx> wrote:
> >>
> >> Hi Raghavendra,
> >>
> >> On 8/17/23 08:30, Raghavendra Rao Ananta wrote:
> >>> From: Reiji Watanabe <reijiw@xxxxxxxxxx>
> >>>
> >>> 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@xxxxxxxxxx>
> >>> Signed-off-by: Raghavendra Rao Ananta <rananta@xxxxxxxxxx>
> >>> ---
> >>>    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;
> >>> +     }
> >>> +     mutex_unlock(&kvm->arch.config_lock);
> >>
> >> Another thing I am just wonder is that should we block any modification
> >> to the pmcr_n after vm start to run? Like add one more checking
> >> kvm_vm_has_ran_once() at the beginning of the set_pmcr() function.
> >>
> > Thanks for bringing it up. Reiji and I discussed about this. Checking
> > for kvm_vm_has_ran_once() will be a good move, however, it will go
> > against the ABI expectations of setting the PMCR. I'd like others to
> > weigh in on this as well. What do you think?
> >
> > Thank you.
> > Raghavendra
>
> Before this change, kvm not allowed userspace to change the pmcr_n, but
> allowed to change the lower ARMV8_PMU_PMCR_MASK bits. With this change,
> we now allow to change the pmcr_n, we should not block the change to
> ARMV8_PMU_PMCR_MASK after vm start to run, but how about we just block
> the change to ARMV8_PMU_PMCR_N after vm start to run?
>
I believe you are referring to the guest trap access part of it
(access_pmcr()). This patch focuses on the userspace access of PMCR
via the KVM_SET_ONE_REG ioctl. Before this patch, KVM did not control
the writes to the reg and userspace was free to write to any bits at
any time.

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