Re: [PATCH v5 17/24] KVM: arm64: Context swap Partitioned PMU guest registers

Oliver Upton
2025-12-09T21:56:00Z

On Tue, Dec 09, 2025 at 08:51:14PM +0000, Colton Lewis wrote:
> +/**
> + * kvm_pmu_load() - Load untrapped PMU registers
> + * @vcpu: Pointer to struct kvm_vcpu
> + *
> + * Load all untrapped PMU registers from the VCPU into the PCPU. Mask
> + * to only bits belonging to guest-reserved counters and leave
> + * host-reserved counters alone in bitmask registers.
> + */
> +void kvm_pmu_load(struct kvm_vcpu *vcpu)
> +{
> + struct arm_pmu *pmu;
> + u64 mask;
> + u8 i;
> + u64 val;
> +

Assert that preemption is disabled.

> + /*
> + * If we aren't using FGT then we are trapping everything
> + * anyway, so no need to bother with the swap.
> + */
> + if (!kvm_vcpu_pmu_use_fgt(vcpu))
> + return;

Uhh... Then how do events count in this case?

The absence of FEAT_FGT shouldn't affect the residence of the guest PMU
context. We just need to handle the extra traps, ideally by reading the
PMU registers directly as a fast path exit handler.

> + pmu = vcpu->kvm->arch.arm_pmu;
> +
> + for (i = 0; i < pmu->hpmn_max; i++) {
> + val = __vcpu_sys_reg(vcpu, PMEVCNTR0_EL0 + i);
> + write_pmevcntrn(i, val);
> + }
> +
> + val = __vcpu_sys_reg(vcpu, PMCCNTR_EL0);
> + write_pmccntr(val);
> +
> + val = __vcpu_sys_reg(vcpu, PMUSERENR_EL0);
> + write_pmuserenr(val);

What about the host's value for PMUSERENR?

> + val = __vcpu_sys_reg(vcpu, PMSELR_EL0);
> + write_pmselr(val);

PMSELR_EL0 needs to be switched late, e.g. at sysreg_restore_guest_state_vhe().
Even though the host doesn't currently use the selector-based accessor,
I'd prefer we not load things that'd affect the host context until we're
about to enter the guest.

> + /* Save only the stateful writable bits. */
> + val = __vcpu_sys_reg(vcpu, PMCR_EL0);
> + mask = ARMV8_PMU_PMCR_MASK &
> + ~(ARMV8_PMU_PMCR_P | ARMV8_PMU_PMCR_C);
> + write_pmcr(val & mask);
> +
> + /*
> + * When handling these:
> + * 1. Apply only the bits for guest counters (indicated by mask)
> + * 2. Use the different registers for set and clear
> + */
> + mask = kvm_pmu_guest_counter_mask(pmu);
> +
> + val = __vcpu_sys_reg(vcpu, PMCNTENSET_EL0);
> + write_pmcntenset(val & mask);
> + write_pmcntenclr(~val & mask);
> +
> + val = __vcpu_sys_reg(vcpu, PMINTENSET_EL1);
> + write_pmintenset(val & mask);
> + write_pmintenclr(~val & mask);

Is this safe? What happens if we put the PMU into an overflow condition?

> +}
> +
> +/**
> + * kvm_pmu_put() - Put untrapped PMU registers
> + * @vcpu: Pointer to struct kvm_vcpu
> + *
> + * Put all untrapped PMU registers from the VCPU into the PCPU. Mask
> + * to only bits belonging to guest-reserved counters and leave
> + * host-reserved counters alone in bitmask registers.
> + */
> +void kvm_pmu_put(struct kvm_vcpu *vcpu)
> +{
> + struct arm_pmu *pmu;
> + u64 mask;
> + u8 i;
> + u64 val;
> +
> + /*
> + * If we aren't using FGT then we are trapping everything
> + * anyway, so no need to bother with the swap.
> + */
> + if (!kvm_vcpu_pmu_use_fgt(vcpu))
> + return;
> +
> + pmu = vcpu->kvm->arch.arm_pmu;
> +
> + for (i = 0; i < pmu->hpmn_max; i++) {
> + val = read_pmevcntrn(i);
> + __vcpu_assign_sys_reg(vcpu, PMEVCNTR0_EL0 + i, val);
> + }
> +
> + val = read_pmccntr();
> + __vcpu_assign_sys_reg(vcpu, PMCCNTR_EL0, val);
> +
> + val = read_pmuserenr();
> + __vcpu_assign_sys_reg(vcpu, PMUSERENR_EL0, val);
> +
> + val = read_pmselr();
> + __vcpu_assign_sys_reg(vcpu, PMSELR_EL0, val);
> +
> + val = read_pmcr();
> + __vcpu_assign_sys_reg(vcpu, PMCR_EL0, val);
> +
> + /* Mask these to only save the guest relevant bits. */
> + mask = kvm_pmu_guest_counter_mask(pmu);
> +
> + val = read_pmcntenset();
> + __vcpu_assign_sys_reg(vcpu, PMCNTENSET_EL0, val & mask);
> +
> + val = read_pmintenset();
> + __vcpu_assign_sys_reg(vcpu, PMINTENSET_EL1, val & mask);

What if the PMU is in an overflow state at this point?

Thanks,
Oliver