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

Oliver Upton
2023-09-15T19:23:42Z

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