Re: [PATCH v6 4/6] KVM: arm64: Use per guest ID register for ID_AA64DFR0_EL1.PMUVer

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Hi Jing,

On Mon, Apr 24, 2023 at 12:07:31PM -0700, Jing Zhang wrote:
> Hi Reiji,
> 
> On Tue, Apr 18, 2023 at 8:40 PM Reiji Watanabe <reijiw@xxxxxxxxxx> wrote:
> >
> > Hi Jing,
> >
> > On Tue, Apr 04, 2023 at 03:53:42AM +0000, Jing Zhang wrote:
> > > With per guest ID registers, PMUver settings from userspace
> > > can be stored in its corresponding ID register.
> > >
> > > No functional change intended.
> > >
> > > Signed-off-by: Jing Zhang <jingzhangos@xxxxxxxxxx>
> > > ---
> > >  arch/arm64/include/asm/kvm_host.h | 11 +++----
> > >  arch/arm64/kvm/arm.c              |  6 ----
> > >  arch/arm64/kvm/id_regs.c          | 50 ++++++++++++++++++++++++-------
> > >  include/kvm/arm_pmu.h             |  5 ++--
> > >  4 files changed, 49 insertions(+), 23 deletions(-)
> > >
> > > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > > index 67a55177fd83..da46a2729581 100644
> > > --- a/arch/arm64/include/asm/kvm_host.h
> > > +++ b/arch/arm64/include/asm/kvm_host.h
> > > @@ -237,6 +237,12 @@ struct kvm_arch {
> > >  #define KVM_ARCH_FLAG_EL1_32BIT                              4
> > >       /* PSCI SYSTEM_SUSPEND enabled for the guest */
> > >  #define KVM_ARCH_FLAG_SYSTEM_SUSPEND_ENABLED         5
> > > +     /*
> > > +      * AA64DFR0_EL1.PMUver was set as ID_AA64DFR0_EL1_PMUVer_IMP_DEF
> > > +      * or DFR0_EL1.PerfMon was set as ID_DFR0_EL1_PerfMon_IMPDEF from
> > > +      * userspace for VCPUs without PMU.
> > > +      */
> > > +#define KVM_ARCH_FLAG_VCPU_HAS_IMP_DEF_PMU           6
> > >
> > >       unsigned long flags;
> > >
> > > @@ -249,11 +255,6 @@ struct kvm_arch {
> > >
> > >       cpumask_var_t supported_cpus;
> > >
> > > -     struct {
> > > -             u8 imp:4;
> > > -             u8 unimp:4;
> > > -     } dfr0_pmuver;
> > > -
> > >       /* Hypercall features firmware registers' descriptor */
> > >       struct kvm_smccc_features smccc_feat;
> > >
> > > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > > index 64e1c19e5a9b..3fe28d545b54 100644
> > > --- a/arch/arm64/kvm/arm.c
> > > +++ b/arch/arm64/kvm/arm.c
> > > @@ -138,12 +138,6 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
> > >       kvm_arm_init_hypercalls(kvm);
> > >       kvm_arm_init_id_regs(kvm);
> > >
> > > -     /*
> > > -      * Initialise the default PMUver before there is a chance to
> > > -      * create an actual PMU.
> > > -      */
> > > -     kvm->arch.dfr0_pmuver.imp = kvm_arm_pmu_get_pmuver_limit();
> > > -
> > >       return 0;
> > >
> > >  err_free_cpumask:
> > > diff --git a/arch/arm64/kvm/id_regs.c b/arch/arm64/kvm/id_regs.c
> > > index 291311b1ecca..6f65d30693fe 100644
> > > --- a/arch/arm64/kvm/id_regs.c
> > > +++ b/arch/arm64/kvm/id_regs.c
> > > @@ -21,9 +21,12 @@
> > >  static u8 vcpu_pmuver(const struct kvm_vcpu *vcpu)
> > >  {
> > >       if (kvm_vcpu_has_pmu(vcpu))
> > > -             return vcpu->kvm->arch.dfr0_pmuver.imp;
> > > +             return FIELD_GET(ARM64_FEATURE_MASK(ID_AA64DFR0_EL1_PMUVer),
> > > +                              IDREG(vcpu->kvm, SYS_ID_AA64DFR0_EL1));
> > > +     else if (test_bit(KVM_ARCH_FLAG_VCPU_HAS_IMP_DEF_PMU, &vcpu->kvm->arch.flags))
> > > +             return ID_AA64DFR0_EL1_PMUVer_IMP_DEF;
> > >
> > > -     return vcpu->kvm->arch.dfr0_pmuver.unimp;
> > > +     return 0;
> > >  }
> > >
> > >  static u8 perfmon_to_pmuver(u8 perfmon)
> > > @@ -254,10 +257,20 @@ static int set_id_aa64dfr0_el1(struct kvm_vcpu *vcpu,
> > >       if (val)
> > >               return -EINVAL;
> > >
> > > -     if (valid_pmu)
> > > -             vcpu->kvm->arch.dfr0_pmuver.imp = pmuver;
> > > -     else
> > > -             vcpu->kvm->arch.dfr0_pmuver.unimp = pmuver;
> > > +     if (valid_pmu) {
> > > +             mutex_lock(&vcpu->kvm->arch.config_lock);
> > > +             IDREG(vcpu->kvm, SYS_ID_AA64DFR0_EL1) &= ~ID_AA64DFR0_EL1_PMUVer_MASK;
> > > +             IDREG(vcpu->kvm, SYS_ID_AA64DFR0_EL1) |= FIELD_PREP(ID_AA64DFR0_EL1_PMUVer_MASK,
> > > +                                                                 pmuver);
> > > +
> > > +             IDREG(vcpu->kvm, SYS_ID_DFR0_EL1) &= ~ID_DFR0_EL1_PerfMon_MASK;
> > > +             IDREG(vcpu->kvm, SYS_ID_DFR0_EL1) |= FIELD_PREP(ID_DFR0_EL1_PerfMon_MASK,
> > > +                                                             pmuver_to_perfmon(pmuver));
> >
> > As those could be read without acquiring the lock, I don't think
> > we should expose the intermediate state of the register values.
> I will protect all reads/writes to KVM scope emulated ID registers
> with the lock.

Or I think we could resolve it by writing the new value atomically
(copy the value to a local variable, set the local variable to the
new value, and update the ID_REG value with WRITE_ONCE).

Thank you,
Reiji


> >
> >
> > > +             mutex_unlock(&vcpu->kvm->arch.config_lock);
> > > +     } else {
> > > +             assign_bit(KVM_ARCH_FLAG_VCPU_HAS_IMP_DEF_PMU, &vcpu->kvm->arch.flags,
> > > +                        pmuver == ID_AA64DFR0_EL1_PMUVer_IMP_DEF);
> > > +     }
> > >
> > >       return 0;
> > >  }
> > > @@ -294,10 +307,19 @@ static int set_id_dfr0_el1(struct kvm_vcpu *vcpu,
> > >       if (val)
> > >               return -EINVAL;
> > >
> > > -     if (valid_pmu)
> > > -             vcpu->kvm->arch.dfr0_pmuver.imp = perfmon_to_pmuver(perfmon);
> > > -     else
> > > -             vcpu->kvm->arch.dfr0_pmuver.unimp = perfmon_to_pmuver(perfmon);
> > > +     if (valid_pmu) {
> > > +             mutex_lock(&vcpu->kvm->arch.config_lock);
> > > +             IDREG(vcpu->kvm, SYS_ID_DFR0_EL1) &= ~ID_DFR0_EL1_PerfMon_MASK;
> > > +             IDREG(vcpu->kvm, SYS_ID_DFR0_EL1) |= FIELD_PREP(ID_DFR0_EL1_PerfMon_MASK, perfmon);
> > > +
> > > +             IDREG(vcpu->kvm, SYS_ID_AA64DFR0_EL1) &= ~ID_AA64DFR0_EL1_PMUVer_MASK;
> > > +             IDREG(vcpu->kvm, SYS_ID_AA64DFR0_EL1) |= FIELD_PREP(ID_AA64DFR0_EL1_PMUVer_MASK,
> > > +                                                                 perfmon_to_pmuver(perfmon));
> >
> > I have the same comment as set_id_aa64dfr0_el1().
> >
> > Thank you,
> > Reiji
> >
> > > +             mutex_unlock(&vcpu->kvm->arch.config_lock);
> > > +     } else {
> > > +             assign_bit(KVM_ARCH_FLAG_VCPU_HAS_IMP_DEF_PMU, &vcpu->kvm->arch.flags,
> > > +                        perfmon == ID_DFR0_EL1_PerfMon_IMPDEF);
> > > +     }
> > >
> > >       return 0;
> > >  }
> > > @@ -503,4 +525,12 @@ void kvm_arm_init_id_regs(struct kvm *kvm)
> > >       }
> > >
> > >       IDREG(kvm, SYS_ID_AA64PFR0_EL1) = val;
> > > +
> > > +     /*
> > > +      * Initialise the default PMUver before there is a chance to
> > > +      * create an actual PMU.
> > > +      */
> > > +     IDREG(kvm, SYS_ID_AA64DFR0_EL1) &= ~ARM64_FEATURE_MASK(ID_AA64DFR0_EL1_PMUVer);
> > > +     IDREG(kvm, SYS_ID_AA64DFR0_EL1) |= FIELD_PREP(ARM64_FEATURE_MASK(ID_AA64DFR0_EL1_PMUVer),
> > > +                                                   kvm_arm_pmu_get_pmuver_limit());
> > >  }
> > > diff --git a/include/kvm/arm_pmu.h b/include/kvm/arm_pmu.h
> > > index 628775334d5e..856ac59b6821 100644
> > > --- a/include/kvm/arm_pmu.h
> > > +++ b/include/kvm/arm_pmu.h
> > > @@ -92,8 +92,9 @@ void kvm_vcpu_pmu_restore_host(struct kvm_vcpu *vcpu);
> > >  /*
> > >   * Evaluates as true when emulating PMUv3p5, and false otherwise.
> > >   */
> > > -#define kvm_pmu_is_3p5(vcpu)                                         \
> > > -     (vcpu->kvm->arch.dfr0_pmuver.imp >= ID_AA64DFR0_EL1_PMUVer_V3P5)
> > > +#define kvm_pmu_is_3p5(vcpu)                                                                 \
> > > +      (FIELD_GET(ARM64_FEATURE_MASK(ID_AA64DFR0_EL1_PMUVer),                                 \
> > > +              IDREG(vcpu->kvm, SYS_ID_AA64DFR0_EL1)) >= ID_AA64DFR0_EL1_PMUVer_V3P5)
> > >
> > >  u8 kvm_arm_pmu_get_pmuver_limit(void);
> > >
> > > --
> > > 2.40.0.348.gf938b09366-goog
> > >
> Thanks,
> Jing



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