Re: [PATCH v2 1/3] KVM: arm64: PMU: Fix SET_ONE_REG for vPMC regs

[Marc Zyngier]

On Fri, 07 Mar 2025 10:55:28 +0000,
Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> 
> Reload the perf event when setting the vPMU counter (vPMC) registers
> (PMCCNTR_EL0 and PMEVCNTR<n>_EL0). This is a change corresponding to
> commit 9228b26194d1 ("KVM: arm64: PMU: Fix GET_ONE_REG
> for vPMC regs to return the current value") but for SET_ONE_REG.
> 
> Values of vPMC registers are saved in sysreg files on certain occasions.
> These saved values don't represent the current values of the vPMC
> registers if the perf events for the vPMCs count events after the save.
> The current values of those registers are the sum of the sysreg file
> value and the current perf event counter value.  But, when userspace
> writes those registers (using KVM_SET_ONE_REG), KVM only updates the
> sysreg file value and leaves the current perf event counter value as is.
> 
> Fix this by releasing the current perf event and trigger recreating one
> with KVM_REQ_RELOAD_PMU.
> 
> Fixes: 051ff581ce70 ("arm64: KVM: Add access handler for event counter register")
> Signed-off-by: Akihiko Odaki <akihiko.odaki@xxxxxxxxxx>
> ---
>  arch/arm64/kvm/pmu-emul.c | 16 ++++++++++++++++
>  arch/arm64/kvm/sys_regs.c | 20 +++++++++++++++++++-
>  include/kvm/arm_pmu.h     |  1 +
>  3 files changed, 36 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> index e3e82b66e2268d37d5e2630e47ddf085a6846e1c..1402cce5625bffa706aabe5e6121d1f3817a0aaf 100644
> --- a/arch/arm64/kvm/pmu-emul.c
> +++ b/arch/arm64/kvm/pmu-emul.c
> @@ -191,6 +191,22 @@ void kvm_pmu_set_counter_value(struct kvm_vcpu *vcpu, u64 select_idx, u64 val)
>  	kvm_pmu_set_pmc_value(kvm_vcpu_idx_to_pmc(vcpu, select_idx), val, false);
>  }
>  
> +/**
> + * kvm_pmu_set_counter_value_user - set PMU counter value from user
> + * @vcpu: The vcpu pointer
> + * @select_idx: The counter index
> + * @val: The counter value
> + */
> +void kvm_pmu_set_counter_value_user(struct kvm_vcpu *vcpu, u64 select_idx, u64 val)
> +{
> +	if (!kvm_vcpu_has_pmu(vcpu))
> +		return;

How can this occur? It seems to me that we only get here from a
register that has .visibility == pmu_visibility().

Or is there another way to reach this function?

> +
> +	kvm_pmu_release_perf_event(kvm_vcpu_idx_to_pmc(vcpu, select_idx));
> +	__vcpu_sys_reg(vcpu, counter_index_to_reg(select_idx)) = val;
> +	kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
> +}
> +
>  /**
>   * kvm_pmu_release_perf_event - remove the perf event
>   * @pmc: The PMU counter pointer
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 42791971f75887796afab905cc12f49fead39e10..27418dac791df9a89124f867879e899db175e506 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1035,6 +1035,22 @@ static int get_pmu_evcntr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
>  	return 0;
>  }
>  
> +static int set_pmu_evcntr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
> +			  u64 val)
> +{
> +	u64 idx;
> +
> +	if (r->CRn == 9 && r->CRm == 13 && r->Op2 == 0)
> +		/* PMCCNTR_EL0 */
> +		idx = ARMV8_PMU_CYCLE_IDX;
> +	else
> +		/* PMEVCNTRn_EL0 */
> +		idx = ((r->CRm & 3) << 3) | (r->Op2 & 7);
> +
> +	kvm_pmu_set_counter_value_user(vcpu, idx, val);
> +	return 0;
> +}
> +
>  static bool access_pmu_evcntr(struct kvm_vcpu *vcpu,
>  			      struct sys_reg_params *p,
>  			      const struct sys_reg_desc *r)
> @@ -1328,6 +1344,7 @@ static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
>  #define PMU_PMEVCNTR_EL0(n)						\
>  	{ PMU_SYS_REG(PMEVCNTRn_EL0(n)),				\
>  	  .reset = reset_pmevcntr, .get_user = get_pmu_evcntr,		\
> +	  .set_user = set_pmu_evcntr,					\
>  	  .access = access_pmu_evcntr, .reg = (PMEVCNTR0_EL0 + n), }
>  
>  /* Macro to expand the PMEVTYPERn_EL0 register */
> @@ -2682,7 +2699,8 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  	  .access = access_pmceid, .reset = NULL },
>  	{ PMU_SYS_REG(PMCCNTR_EL0),
>  	  .access = access_pmu_evcntr, .reset = reset_unknown,
> -	  .reg = PMCCNTR_EL0, .get_user = get_pmu_evcntr},
> +	  .reg = PMCCNTR_EL0, .get_user = get_pmu_evcntr,
> +	  .set_user = set_pmu_evcntr },
>  	{ PMU_SYS_REG(PMXEVTYPER_EL0),
>  	  .access = access_pmu_evtyper, .reset = NULL },
>  	{ PMU_SYS_REG(PMXEVCNTR_EL0),
> diff --git a/include/kvm/arm_pmu.h b/include/kvm/arm_pmu.h
> index 28b380ad8dfa942c4275e0c7ed3535d309b81b2f..9c062756ebfad5ea555362154459ffe9f8311c6d 100644
> --- a/include/kvm/arm_pmu.h
> +++ b/include/kvm/arm_pmu.h
> @@ -41,6 +41,7 @@ bool kvm_supports_guest_pmuv3(void);
>  #define kvm_arm_pmu_irq_initialized(v)	((v)->arch.pmu.irq_num >= VGIC_NR_SGIS)
>  u64 kvm_pmu_get_counter_value(struct kvm_vcpu *vcpu, u64 select_idx);
>  void kvm_pmu_set_counter_value(struct kvm_vcpu *vcpu, u64 select_idx, u64 val);
> +void kvm_pmu_set_counter_value_user(struct kvm_vcpu *vcpu, u64 select_idx, u64 val);
>  u64 kvm_pmu_valid_counter_mask(struct kvm_vcpu *vcpu);
>  u64 kvm_pmu_get_pmceid(struct kvm_vcpu *vcpu, bool pmceid1);
>  void kvm_pmu_vcpu_init(struct kvm_vcpu *vcpu);

Other than the nit above, looks good to me.

	M.

-- 
Without deviation from the norm, progress is not possible.