Re: [PATCH v7 02/12] KVM: arm64: PMU: Set the default PMU for the guest before vCPU reset

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

Hi Raghu,

On Mon, Oct 09, 2023 at 11:08:48PM +0000, Raghavendra Rao Ananta wrote:
> From: Reiji Watanabe <reijiw@google.com>
> 
> The following patches will use the number of counters information
> from the arm_pmu and use this to set the PMCR.N for the guest
> during vCPU reset. However, since the guest is not associated
> with any arm_pmu until userspace configures the vPMU device
> attributes, and a reset can happen before this event, assign a
> default PMU to the guest just before doing the reset.
> 
> Signed-off-by: Reiji Watanabe <reijiw@google.com>
> Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
> ---
>  arch/arm64/kvm/arm.c      | 20 ++++++++++++++++++++
>  arch/arm64/kvm/pmu-emul.c | 12 ++----------
>  include/kvm/arm_pmu.h     |  6 ++++++
>  3 files changed, 28 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 78b0970eb8e6..708a53b70a7b 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -1313,6 +1313,23 @@ static bool kvm_vcpu_init_changed(struct kvm_vcpu *vcpu,
>  			     KVM_VCPU_MAX_FEATURES);
>  }
>  
> +static int kvm_vcpu_set_pmu(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm *kvm = vcpu->kvm;
> +
> +	if (!kvm_arm_support_pmu_v3())
> +		return -EINVAL;

This check is pointless; the vCPU feature flags have been sanitised at
this point, and a requirement of having PMUv3 is that this predicate is
true.

> +	/*
> +	 * When the vCPU has a PMU, but no PMU is set for the guest
> +	 * yet, set the default one.
> +	 */
> +	if (unlikely(!kvm->arch.arm_pmu))
> +		return kvm_arm_set_default_pmu(kvm);
> +
> +	return 0;
> +}
> +

Apologies, I believe I was unclear last time around as to what I was
wanting here. Let's call this thing kvm_setup_vcpu() such that we can
add other one-time setup activities to it in the future.

Something like:

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 96641e442039..4896a44108e0 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -1265,19 +1265,17 @@ static bool kvm_vcpu_init_changed(struct kvm_vcpu *vcpu,
 			     KVM_VCPU_MAX_FEATURES);
 }
 
-static int kvm_vcpu_set_pmu(struct kvm_vcpu *vcpu)
+static int kvm_setup_vcpu(struct kvm_vcpu *vcpu)
 {
 	struct kvm *kvm = vcpu->kvm;
 
-	if (!kvm_arm_support_pmu_v3())
-		return -EINVAL;
-
 	/*
 	 * When the vCPU has a PMU, but no PMU is set for the guest
 	 * yet, set the default one.
 	 */
-	if (unlikely(!kvm->arch.arm_pmu))
-		return kvm_arm_set_default_pmu(kvm);
+	if (kvm_vcpu_has_pmu(vcpu) && !kvm->arch.arm_pmu &&
+	    kvm_arm_set_default_pmu(kvm))
+		return -EINVAL;
 
 	return 0;
 }
@@ -1297,7 +1295,8 @@ static int __kvm_vcpu_set_target(struct kvm_vcpu *vcpu,
 
 	bitmap_copy(kvm->arch.vcpu_features, &features, KVM_VCPU_MAX_FEATURES);
 
-	if (kvm_vcpu_has_pmu(vcpu) && kvm_vcpu_set_pmu(vcpu))
+	ret = kvm_setup_vcpu(vcpu);
+	if (ret)
 		goto out_unlock;
 
 	/* Now we know what it is, we can reset it. */

-- 
Thanks,
Oliver