Re: [PATCH v5 14/24] KVM: arm64: Write fast path PMU register handlers

Oliver Upton
2025-12-17T00:38:58Z

On Tue, Dec 09, 2025 at 08:51:11PM +0000, Colton Lewis wrote:
> diff --git a/arch/arm64/include/asm/arm_pmuv3.h b/arch/arm64/include/asm/arm_pmuv3.h
> index 3e25c0313263c..41ec6730ebc62 100644
> --- a/arch/arm64/include/asm/arm_pmuv3.h
> +++ b/arch/arm64/include/asm/arm_pmuv3.h
> @@ -39,6 +39,16 @@ static inline unsigned long read_pmevtypern(int n)
> return 0;
> }
>
> +static inline void write_pmxevcntr(u64 val)
> +{
> + write_sysreg(val, pmxevcntr_el0);
> +}
> +
> +static inline u64 read_pmxevcntr(void)
> +{
> + return read_sysreg(pmxevcntr_el0);
> +}
> +
> static inline unsigned long read_pmmir(void)
> {
> return read_cpuid(PMMIR_EL1);
> @@ -105,21 +115,41 @@ static inline void write_pmcntenset(u64 val)
> write_sysreg(val, pmcntenset_el0);
> }
>
> +static inline u64 read_pmcntenset(void)
> +{
> + return read_sysreg(pmcntenset_el0);
> +}
> +
> static inline void write_pmcntenclr(u64 val)
> {
> write_sysreg(val, pmcntenclr_el0);
> }
>
> +static inline u64 read_pmcntenclr(void)
> +{
> + return read_sysreg(pmcntenclr_el0);
> +}
> +
> static inline void write_pmintenset(u64 val)
> {
> write_sysreg(val, pmintenset_el1);
> }
>
> +static inline u64 read_pmintenset(void)
> +{
> + return read_sysreg(pmintenset_el1);
> +}
> +
> static inline void write_pmintenclr(u64 val)
> {
> write_sysreg(val, pmintenclr_el1);
> }
>
> +static inline u64 read_pmintenclr(void)
> +{
> + return read_sysreg(pmintenclr_el1);
> +}
> +
> static inline void write_pmccfiltr(u64 val)
> {
> write_sysreg(val, pmccfiltr_el0);
> @@ -160,11 +190,16 @@ static inline u64 read_pmovsclr(void)
> return read_sysreg(pmovsclr_el0);
> }
>
> -static inline void write_pmuserenr(u32 val)
> +static inline void write_pmuserenr(u64 val)
> {
> write_sysreg(val, pmuserenr_el0);
> }
>
> +static inline u64 read_pmuserenr(void)
> +{
> + return read_sysreg(pmuserenr_el0);
> +}
> +
> static inline void write_pmuacr(u64 val)
> {
> write_sysreg_s(val, SYS_PMUACR_EL1);

I wouldn't bother with adding accessors to the PMUv3 driver header, this
gets a bit confusing when the 32-bit counterpart lacks matching definitions.
Additionally, the part of KVM you're modifying is very much an
"internal" part meant to run in a not-quite-kernel context.

Considering this, I'd prefer KVM directly accessed the PMU registers to
avoid the possibility of taking some instrumented codepath in the
future.

> + if (!kvm_vcpu_pmu_is_partitioned(vcpu)
> + || pmu_access_el0_disabled(vcpu))

Always put operators on the preceding line for line continuations.

Also, don't call pmu_access_el0_disabled() from here. It can potentially
do a full emulated exception entry even though the vCPU is in an
extremely inconsistent state (i.e. haven't returned to kernel context
yet). Isn't the current value for PMUSERENR_EL0 on the CPU instead of in
the vCPU's saved context anyway?

The fast-path accessors really need to be *just* accessors, reading
state from the CPU in the unfortunate situation that a TPM trap has been
forced upon us.

> + case SYS_PMXEVCNTR_EL0:
> + idx = FIELD_GET(PMSELR_EL0_SEL, read_pmselr());
> +
> + if (pmu_access_event_counter_el0_disabled(vcpu))
> + return false;
> +
> + if (!pmu_counter_idx_valid(vcpu, idx))
> + return false;
> +
> + ret = handle_pmu_reg(vcpu, &p, PMEVCNTR0_EL0 + idx, rt, val,
> + &read_pmxevcntr, &write_pmxevcntr);
> + break;

It is a bit odd to handle the muxing for finding the in-memory value yet
using the selector-based register for hardware.

> + case SYS_PMEVCNTRn_EL0(0) ... SYS_PMEVCNTRn_EL0(30):
> + idx = ((p.CRm & 3) << 3) | (p.Op2 & 7);
> +
> + if (pmu_access_event_counter_el0_disabled(vcpu))
> + return false;
> +
> + if (!pmu_counter_idx_valid(vcpu, idx))
> + return false;
> +
> + if (p.is_write) {
> + write_pmevcntrn(idx, val);
> + __vcpu_assign_sys_reg(vcpu, PMEVCNTR0_EL0 + idx, val);
> + } else {
> + vcpu_set_reg(vcpu, rt, read_pmevcntrn(idx));
> + }
> +
> + ret = true;
> + break;

Can't both of these cases share a helper once you've worked out the
index? Same for PMEVTYPERn_EL0.

> + default:
> + ret = false;
> + }
> +
> + if (ret)
> + __kvm_skip_instr(vcpu);
> +
> + return ret;
> +}
> +
> static inline bool kvm_hyp_handle_sysreg(struct kvm_vcpu *vcpu, u64 *exit_code)
> {
> if (cpus_have_final_cap(ARM64_WORKAROUND_CAVIUM_TX2_219_TVM) &&
> @@ -785,6 +983,9 @@ static inline bool kvm_hyp_handle_sysreg(struct kvm_vcpu *vcpu, u64 *exit_code)
> if (kvm_handle_cntxct(vcpu))
> return true;
>
> + if (kvm_hyp_handle_pmu_regs(vcpu))
> + return true;
> +

Since the whole partitioned PMU feature is constrained to VHE-only you
should call this from kvm_hyp_handle_sysreg_vhe().

> +
> +bool check_pmu_access_disabled(struct kvm_vcpu *vcpu, u64 flags)
> +{
> + u64 reg = __vcpu_sys_reg(vcpu, PMUSERENR_EL0);
> + bool enabled = (reg & flags) || vcpu_mode_priv(vcpu);
> +
> + if (!enabled)
> + kvm_inject_undefined(vcpu);
> +
> + return !enabled;
> +}
> +
> +bool pmu_access_el0_disabled(struct kvm_vcpu *vcpu)
> +{
> + return check_pmu_access_disabled(vcpu, ARMV8_PMU_USERENR_EN);
> +}
> +
> +bool pmu_counter_idx_valid(struct kvm_vcpu *vcpu, u64 idx)
> +{
> + u64 pmcr, val;
> +
> + pmcr = kvm_vcpu_read_pmcr(vcpu);
> + val = FIELD_GET(ARMV8_PMU_PMCR_N, pmcr);
> + if (idx >= val && idx != ARMV8_PMU_CYCLE_IDX) {
> + kvm_inject_undefined(vcpu);
> + return false;
> + }
> +
> + return true;
> +}
> +
> +bool pmu_access_cycle_counter_el0_disabled(struct kvm_vcpu *vcpu)
> +{
> + return check_pmu_access_disabled(vcpu, ARMV8_PMU_USERENR_CR | ARMV8_PMU_USERENR_EN);
> +}
> +
> +bool pmu_access_event_counter_el0_disabled(struct kvm_vcpu *vcpu)
> +{
> + return check_pmu_access_disabled(vcpu, ARMV8_PMU_USERENR_ER | ARMV8_PMU_USERENR_EN);
> +}

Refactorings need to happen in a separate patch.

Thanks,
Oliver