Subject: Re: [PATCH v3 2/4] KVM: arm64: Keep a list of probed PMUs

From: Marc Zyngier <[email-masked]>
Date: Tue, 14 Dec 2021 12:30:30 +0000

On Mon, 13 Dec 2021 15:23:07 +0000,
Alexandru Elisei <[email-masked]> wrote:
> 
> The ARM PMU driver calls kvm_host_pmu_init() after probing to tell KVM that
> a hardware PMU is available for guest emulation. Heterogeneous systems can
> have more than one PMU present, and the callback gets called multiple
> times, once for each of them. Keep track of all the PMUs available to KVM,
> as they're going to be needed later.
> 
> Signed-off-by: Alexandru Elisei <[email-masked]>
> ---
>  arch/arm64/kvm/pmu-emul.c | 25 +++++++++++++++++++++++--
>  include/kvm/arm_pmu.h     |  5 +++++
>  2 files changed, 28 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> index a5e4bbf5e68f..eb4be96f144d 100644
> --- a/arch/arm64/kvm/pmu-emul.c
> +++ b/arch/arm64/kvm/pmu-emul.c
> @@ -7,6 +7,7 @@
>  #include <linux/cpu.h>
>  #include <linux/kvm.h>
>  #include <linux/kvm_host.h>
> +#include <linux/list.h>
>  #include <linux/perf_event.h>
>  #include <linux/perf/arm_pmu.h>
>  #include <linux/uaccess.h>
> @@ -14,6 +15,9 @@
>  #include <kvm/arm_pmu.h>
>  #include <kvm/arm_vgic.h>
>  
> +static LIST_HEAD(arm_pmus);
> +static DEFINE_MUTEX(arm_pmus_lock);
> +
>  static void kvm_pmu_create_perf_event(struct kvm_vcpu *vcpu, u64 select_idx);
>  static void kvm_pmu_update_pmc_chained(struct kvm_vcpu *vcpu, u64 select_idx);
>  static void kvm_pmu_stop_counter(struct kvm_vcpu *vcpu, struct kvm_pmc *pmc);
> @@ -742,9 +746,26 @@ void kvm_pmu_set_counter_event_type(struct kvm_vcpu *vcpu, u64 data,
>  
>  void kvm_host_pmu_init(struct arm_pmu *pmu)
>  {
> -	if (pmu->pmuver != 0 && pmu->pmuver != ID_AA64DFR0_PMUVER_IMP_DEF &&
> -	    !kvm_arm_support_pmu_v3() && !is_protected_kvm_enabled())
> +	struct arm_pmu_entry *entry;
> +
> +	if (pmu->pmuver == 0 || pmu->pmuver == ID_AA64DFR0_PMUVER_IMP_DEF ||
> +	    is_protected_kvm_enabled())
> +		return;
> +
> +	mutex_lock(&arm_pmus_lock);
> +
> +	entry = kmalloc(sizeof(*entry), GFP_KERNEL);
> +	if (!entry)
> +		goto out_unlock;
> +
> +	if (list_empty(&arm_pmus))
>  		static_branch_enable(&kvm_arm_pmu_available);

I find it slightly dodgy that you switch the static key before
actually populating the entry. I'd suggest moving it after the
list_add_tail(), and check on list_is_singular() instead.

> +
> +	entry->arm_pmu = pmu;
> +	list_add_tail(&entry->entry, &arm_pmus);
> +
> +out_unlock:
> +	mutex_unlock(&arm_pmus_lock);
>  }
>  
>  static int kvm_pmu_probe_pmuver(void)
> diff --git a/include/kvm/arm_pmu.h b/include/kvm/arm_pmu.h
> index 90f21898aad8..e249c5f172aa 100644
> --- a/include/kvm/arm_pmu.h
> +++ b/include/kvm/arm_pmu.h
> @@ -36,6 +36,11 @@ struct kvm_pmu {
>  	struct irq_work overflow_work;
>  };
>  
> +struct arm_pmu_entry {
> +	struct list_head entry;
> +	struct arm_pmu *arm_pmu;
> +};
> +
>  #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);

Thanks,

	M.

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

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