Re: [PATCH v5 01/12] KVM: arm64: PMU: Introduce a helper to set the guest's PMU

[Oliver Upton <oliver.upton@linux.dev>]

Hi Raghu,

On Thu, Aug 17, 2023 at 12:30:18AM +0000, Raghavendra Rao Ananta wrote:
> From: Reiji Watanabe <reijiw@google.com>
> 
> Introduce a new helper function to set the guest's PMU
> (kvm->arch.arm_pmu), and use it when the guest's PMU needs
> to be set. This helper will make it easier for the following
> patches to modify the relevant code.
> 
> No functional change intended.
> 
> Signed-off-by: Reiji Watanabe <reijiw@google.com>
> Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
> ---
>  arch/arm64/kvm/pmu-emul.c | 52 +++++++++++++++++++++++++++------------
>  1 file changed, 36 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> index 5606509724787..0ffd1efa90c07 100644
> --- a/arch/arm64/kvm/pmu-emul.c
> +++ b/arch/arm64/kvm/pmu-emul.c
> @@ -865,6 +865,32 @@ static bool pmu_irq_is_valid(struct kvm *kvm, int irq)
>  	return true;
>  }
>  
> +static int kvm_arm_set_vm_pmu(struct kvm *kvm, struct arm_pmu *arm_pmu)
> +{
> +	lockdep_assert_held(&kvm->arch.config_lock);
> +
> +	if (!arm_pmu) {
> +		/*
> +		 * No PMU set, get the default one.
> +		 *
> +		 * The observant among you will notice that the supported_cpus
> +		 * mask does not get updated for the default PMU even though it
> +		 * is quite possible the selected instance supports only a
> +		 * subset of cores in the system. This is intentional, and
> +		 * upholds the preexisting behavior on heterogeneous systems
> +		 * where vCPUs can be scheduled on any core but the guest
> +		 * counters could stop working.
> +		 */
> +		arm_pmu = kvm_pmu_probe_armpmu();
> +		if (!arm_pmu)
> +			return -ENODEV;
> +	}
> +
> +	kvm->arch.arm_pmu = arm_pmu;
> +
> +	return 0;
> +}
> +

I'm not too big of a fan of adding the 'default' path to this helper.
I'd prefer it if kvm_arm_set_vm_pmu() does all the necessary
initialization for a valid pmu instance. You then avoid introducing
unexpected error handling where it didn't exist before.

  static void kvm_arm_set_pmu(struct kvm *kvm, struct arm_pmu *arm_pmu)
  {
  	lockdep_assert_held(&kvm->arch.config_lock);

	kvm->arch.arm_pmu = arm_pmu;
  }

  /*
   * Blurb about default PMUs I'm too lazy to copy/paste
   */
  static int kvm_arm_set_default_pmu(struct kvm *kvm)
  {
  	struct arm_pmu *arm_pmu = kvm_pmu_probe_armpmu();

	if (!arm_pmu)
		return -ENODEV;

	kvm_arm_set_pmu(kvm, arm_pmu);
	return 0;
  }

-- 
Thanks,
Oliver

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