Re: [PATCH v3 1/4] KVM: arm64: Expose PMMIR_EL1.SLOTS under strict PMUv3 UAPI

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] The implementation of `KVM_ARM_VCPU_PMU_V3_STRICT` introduces two logic errors that completely break the new feature and its API contract:
1. `KVM_VCPU_MAX_FEATURES` was not increased from 9 to 10 to accommodate the new flag (which is defined as 9). As a result, `KVM_VCPU_VALID_FEATURES` excludes bit 9, causing `kvm_vcpu_init_check_features()` to unconditionally reject `KVM_ARM_VCPU_INIT` ioctls with `-ENOENT` when this feature is requested.
2. Even if the feature validation were fixed, the code fails to enforce the documented 'No default PMU creation' behavior. `kvm_setup_vcpu()` unconditionally calls `kvm_arm_set_default_pmu()` if `kvm_vcpu_has_pmu()` is true, without checking if the strict flag is set. Thus, KVM will always assign a default PMU on initialization, defeating the stated purpose of the flag.
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--- Patch [1]: [PATCH v3 1/4] KVM: arm64: Expose PMMIR_EL1.SLOTS under strict PMUv3 UAPI ---
commit 5883b44965f93aed147ca4ed5fb390ae481daa37
Author: Congkai Tan <congkai@amazon.com>

KVM: arm64: Expose PMMIR_EL1.SLOTS under strict PMUv3 UAPI

This commit introduces a new VCPU feature flag KVM_ARM_VCPU_PMU_V3_STRICT
to prevent KVM from automatically creating a default PMU, and exposes
the PMMIR_EL1.SLOTS value to the guest when a PMU is explicitly selected.

> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
> index 1c13bfa2d38aa..019e5e3d892e6 100644
> --- a/arch/arm64/include/uapi/asm/kvm.h
> +++ b/arch/arm64/include/uapi/asm/kvm.h
> @@ -106,6 +106,7 @@ struct kvm_regs {
>  #define KVM_ARM_VCPU_PTRAUTH_GENERIC	6 /* VCPU uses generic authentication */
>  #define KVM_ARM_VCPU_HAS_EL2		7 /* Support nested virtualization */
>  #define KVM_ARM_VCPU_HAS_EL2_E2H0	8 /* Limit NV support to E2H RES0 */
> +#define KVM_ARM_VCPU_PMU_V3_STRICT	9 /* No default PMU creation */

[Severity: High]
Does KVM_VCPU_MAX_FEATURES need to be updated to 10 to support this new flag?

Currently, KVM_VCPU_MAX_FEATURES in arch/arm64/include/asm/kvm_host.h is 9,
so KVM_VCPU_VALID_FEATURES (BIT(KVM_VCPU_MAX_FEATURES) - 1) only covers bits
0 through 8. When userspace attempts to use this new flag (bit 9),
kvm_vcpu_init_check_features() will reject it with -ENOENT.

Additionally, does this patch actually prevent default PMU creation? 

Looking at kvm_setup_vcpu() in arch/arm64/kvm/arm.c:

	if (kvm_vcpu_has_pmu(vcpu) && !kvm->arch.arm_pmu)
		ret = kvm_arm_set_default_pmu(kvm);

It appears that kvm_arm_set_default_pmu() will still be called on
initialization even if the strict flag is set, since there is no check for
kvm_vcpu_has_pmuv3_strict(vcpu) here.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260722202702.4165917-1-congkai@amazon.com?part=1