Re: [PATCH v5 10/24] KVM: arm64: Set up FGT for Partitioned PMU

Oliver Upton
2025-12-09T21:08:15Z

Hi Colton,

On Tue, Dec 09, 2025 at 08:51:07PM +0000, Colton Lewis wrote:
> In order to gain the best performance benefit from partitioning the
> PMU, utilize fine grain traps (FEAT_FGT and FEAT_FGT2) to avoid
> trapping common PMU register accesses by the guest to remove that
> overhead.
>
> Untrapped:
> * PMCR_EL0
> * PMUSERENR_EL0
> * PMSELR_EL0
> * PMCCNTR_EL0
> * PMCNTEN_EL0
> * PMINTEN_EL1
> * PMEVCNTRn_EL0
>
> These are safe to untrap because writing MDCR_EL2.HPMN as this series
> will do limits the effect of writes to any of these registers to the
> partition of counters 0..HPMN-1. Reads from these registers will not
> leak information from between guests as all these registers are
> context swapped by a later patch in this series. Reads from these
> registers also do not leak any information about the host's hardware
> beyond what is promised by PMUv3.
>
> Trapped:
> * PMOVS_EL0
> * PMEVTYPERn_EL0
> * PMCCFILTR_EL0
> * PMICNTR_EL0
> * PMICFILTR_EL0
> * PMCEIDn_EL0
> * PMMIR_EL1
>
> PMOVS remains trapped so KVM can track overflow IRQs that will need to
> be injected into the guest.
>
> PMICNTR and PMIFILTR remain trapped because KVM is not handling them
> yet.
>
> PMEVTYPERn remains trapped so KVM can limit which events guests can
> count, such as disallowing counting at EL2. PMCCFILTR and PMCIFILTR
> are special cases of the same.
>
> PMCEIDn and PMMIR remain trapped because they can leak information
> specific to the host hardware implementation.
>
> NOTE: This patch temporarily forces kvm_vcpu_pmu_is_partitioned() to
> be false to prevent partial feature activation for easier debugging.
>
> Signed-off-by: Colton Lewis <coltonlewis@google.com>
> ---
> arch/arm64/include/asm/kvm_pmu.h | 33 ++++++++++++++++++++++
> arch/arm64/kvm/config.c | 34 ++++++++++++++++++++--
> arch/arm64/kvm/pmu-direct.c | 48 ++++++++++++++++++++++++++++++++
> 3 files changed, 112 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_pmu.h b/arch/arm64/include/asm/kvm_pmu.h
> index 8887f39c25e60..7297a697a4a62 100644
> --- a/arch/arm64/include/asm/kvm_pmu.h
> +++ b/arch/arm64/include/asm/kvm_pmu.h
> @@ -96,6 +96,23 @@ u64 kvm_pmu_guest_counter_mask(struct arm_pmu *pmu);
> void kvm_pmu_host_counters_enable(void);
> void kvm_pmu_host_counters_disable(void);
>
> +#if !defined(__KVM_NVHE_HYPERVISOR__)
> +bool kvm_vcpu_pmu_is_partitioned(struct kvm_vcpu *vcpu);
> +bool kvm_vcpu_pmu_use_fgt(struct kvm_vcpu *vcpu);
> +#else
> +static inline bool kvm_vcpu_pmu_is_partitioned(struct kvm_vcpu *vcpu)
> +{
> + return false;
> +}
> +
> +static inline bool kvm_vcpu_pmu_use_fgt(struct kvm_vcpu *vcpu)
> +{
> + return false;
> +}
> +#endif
> +u64 kvm_pmu_fgt_bits(void);
> +u64 kvm_pmu_fgt2_bits(void);
> +
> /*
> * Updates the vcpu's view of the pmu events for this cpu.
> * Must be called before every vcpu run after disabling interrupts, to ensure
> @@ -135,6 +152,22 @@ static inline u64 kvm_pmu_get_counter_value(struct kvm_vcpu *vcpu,
> {
> return 0;
> }
> +static inline bool kvm_vcpu_pmu_is_partitioned(struct kvm_vcpu *vcpu)
> +{
> + return false;
> +}
> +static inline bool kvm_vcpu_pmu_use_fgt(struct kvm_vcpu *vcpu)
> +{
> + return false;
> +}
> +static inline u64 kvm_pmu_fgt_bits(void)
> +{
> + return 0;
> +}
> +static inline u64 kvm_pmu_fgt2_bits(void)
> +{
> + return 0;
> +}
> static inline void kvm_pmu_set_counter_value(struct kvm_vcpu *vcpu,
> u64 select_idx, u64 val) {}
> static inline void kvm_pmu_set_counter_value_user(struct kvm_vcpu *vcpu,
> diff --git a/arch/arm64/kvm/config.c b/arch/arm64/kvm/config.c
> index 24bb3f36e9d59..064dc6aa06f76 100644
> --- a/arch/arm64/kvm/config.c
> +++ b/arch/arm64/kvm/config.c
> @@ -6,6 +6,7 @@
>
> #include <linux/kvm_host.h>
> #include <asm/kvm_emulate.h>
> +#include <asm/kvm_pmu.h>
> #include <asm/kvm_nested.h>
> #include <asm/sysreg.h>
>
> @@ -1489,12 +1490,39 @@ static void __compute_hfgwtr(struct kvm_vcpu *vcpu)
> *vcpu_fgt(vcpu, HFGWTR_EL2) |= HFGWTR_EL2_TCR_EL1;
> }
>
> +static void __compute_hdfgrtr(struct kvm_vcpu *vcpu)
> +{
> + __compute_fgt(vcpu, HDFGRTR_EL2);
> +
> + if (kvm_vcpu_pmu_use_fgt(vcpu))
> + *vcpu_fgt(vcpu, HDFGRTR_EL2) |= kvm_pmu_fgt_bits();

Couple of suggestions. I'd rather see this conditioned on
kvm_vcpu_pmu_is_partitioned() and get rid of the FGT predicate. After
all, kvm_vcpu_load_fgt() already checks for the presence of FEAT_FGT
first.

Additionally, I'd prefer that the trap configuration is inline instead
of done in a helper in some other file. Centralizing the FGT
configuration here was very much intentional.

The other reason for doing this is kvm_pmu_fgt_bits() assumes a
'positive' trap polarity, even though there are several cases where FGTs
have a 'negative' priority (i.e. 0 => trap).

Thanks,
Oliver