[PATCH v3 11/14] KVM: arm64: PMU: Allow ID_AA64DFR0_EL1.PMUver to be set from userspace

[Reiji Watanabe]

Hi Marc,

On Mon, Nov 7, 2022 at 1:16 AM Marc Zyngier <maz at kernel.org> wrote:
>
> Allow userspace to write ID_AA64DFR0_EL1, on the condition that only
> the PMUver field can be altered and be at most the one that was
> initially computed for the guest.
>
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> ---
>  arch/arm64/kvm/sys_regs.c | 40 ++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 39 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 7a4cd644b9c0..47c882401f3c 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1247,6 +1247,43 @@ static int set_id_aa64pfr0_el1(struct kvm_vcpu *vcpu,
>         return 0;
>  }
>
> +static int set_id_aa64dfr0_el1(struct kvm_vcpu *vcpu,
> +                              const struct sys_reg_desc *rd,
> +                              u64 val)
> +{
> +       u8 pmuver, host_pmuver;
> +       bool valid_pmu;
> +
> +       host_pmuver = kvm_arm_pmu_get_pmuver_limit();
> +
> +       /*
> +        * Allow AA64DFR0_EL1.PMUver to be set from userspace as long
> +        * as it doesn't promise more than what the HW gives us. We
> +        * allow an IMPDEF PMU though, only if no PMU is supported
> +        * (KVM backward compatibility handling).
> +        */
> +       pmuver = FIELD_GET(ARM64_FEATURE_MASK(ID_AA64DFR0_EL1_PMUVer), val);
> +       if ((pmuver != ID_AA64DFR0_EL1_PMUVer_IMP_DEF && pmuver > host_pmuver) ||
> +           (pmuver != 0 && pmuver < ID_AA64DFR0_EL1_PMUVer_IMP))

Nit: Since this second condition cannot be true (right?), perhaps it might
be rather confusing?  I wasn't able to understand what it meant until
I see the equivalent check in set_id_dfr0_el1() (Maybe just me though:).

Thank you,
Reiji


> +               return -EINVAL;
> +
> +       valid_pmu = (pmuver != 0 && pmuver != ID_AA64DFR0_EL1_PMUVer_IMP_DEF);
> +
> +       /* Make sure view register and PMU support do match */
> +       if (kvm_vcpu_has_pmu(vcpu) != valid_pmu)
> +               return -EINVAL;
> +
> +       /* We can only differ with PMUver, and anything else is an error */
> +       val ^= read_id_reg(vcpu, rd);
> +       val &= ~ARM64_FEATURE_MASK(ID_AA64DFR0_EL1_PMUVer);
> +       if (val)
> +               return -EINVAL;
> +
> +       vcpu->kvm->arch.dfr0_pmuver = pmuver;
> +
> +       return 0;
> +}
> +
>  /*
>   * cpufeature ID register user accessors
>   *
> @@ -1508,7 +1545,8 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>         ID_UNALLOCATED(4,7),
>
>         /* CRm=5 */
> -       ID_SANITISED(ID_AA64DFR0_EL1),
> +       { SYS_DESC(SYS_ID_AA64DFR0_EL1), .access = access_id_reg,
> +         .get_user = get_id_reg, .set_user = set_id_aa64dfr0_el1, },
>         ID_SANITISED(ID_AA64DFR1_EL1),
>         ID_UNALLOCATED(5,2),
>         ID_UNALLOCATED(5,3),
> --
> 2.34.1
>
>