Re: [PATCH v2 11/14] KVM: arm64: PMU: Allow ID_AA64DFR0_EL1.PMUver to be set from userspace

From: Reiji Watanabe <reijiw@google.com>
To: Marc Zyngier <maz@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu,  kvmarm@lists.linux.dev,
	kvm@vger.kernel.org,  James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	 Alexandru Elisei <alexandru.elisei@arm.com>,
	Oliver Upton <oliver.upton@linux.dev>,
	 Ricardo Koller <ricarkol@google.com>
Subject: Re: [PATCH v2 11/14] KVM: arm64: PMU: Allow ID_AA64DFR0_EL1.PMUver to be set from userspace
Date: Wed, 2 Nov 2022 22:31:56 -0700	[thread overview]
Message-ID: <CAAeT=FyiNeRun7oRL83AUkVabUSb9pxL2SS9yZwi1rjFnbhH6g@mail.gmail.com> (raw)
In-Reply-To: <20221028105402.2030192-12-maz@kernel.org>

Hi Marc,

On Fri, Oct 28, 2022 at 4:16 AM Marc Zyngier <maz@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@kernel.org>
> ---
>  arch/arm64/kvm/sys_regs.c | 37 ++++++++++++++++++++++++++++++++++++-
>  1 file changed, 36 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 7a4cd644b9c0..4fa14b4ae2a6 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1247,6 +1247,40 @@ 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;
> +
> +       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).
> +        */

It appears the patch allows userspace to set IMPDEF even
when host_pmuver == 0.  Shouldn't it be allowed only when
host_pmuver == IMPDEF (as before)?
Probably, it may not cause any real problems though.

> +       pmuver = FIELD_GET(ARM64_FEATURE_MASK(ID_AA64DFR0_EL1_PMUVer), val);
> +       if (pmuver != ID_AA64DFR0_EL1_PMUVer_IMP_DEF && pmuver > host_pmuver)
> +               return -EINVAL;
> +
> +       /* We already have a PMU, don't try to disable it... */
> +       if (kvm_vcpu_has_pmu(vcpu) &&
> +           (pmuver == 0 || pmuver == ID_AA64DFR0_EL1_PMUVer_IMP_DEF))
> +               return -EINVAL;

Nit: Perhaps it might be useful to return a different error code for the
above two (new) error cases (I plan to use -E2BIG and -EPERM
respectively for those cases with my ID register series).

Thank you,
Reiji

> +
> +       /* 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 +1542,8 @@ 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
>
>

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