Re: [PATCH v2] KVM: arm64: Register irq_bypass consumers only when direct MSIs are possible

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] KVM: arm64: irq_bypass skipped if vITS is created after
  KVM_IRQFD
--

commit 3a0bd42f42a6d2802fe72bb1bc5c8b719382be8f
Author: Jinqian Yang <yangjinqian1@huawei.com>

KVM: arm64: Register irq_bypass consumers only when direct MSIs are possible

This commit passes struct kvm to kvm_arch_has_irq_bypass() and gates
arm64 irq_bypass consumer registration on vgic_supports_direct_msis(kvm).

> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 97332ffcb1300..da86646b67097 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -1742,9 +1742,9 @@ void kvm_set_vm_id_reg(struct kvm *kvm, u32 reg, u64 val);
>  #define kvm_has_sctlr2(k)				\
>  	(kvm_has_feat((k), ID_AA64MMFR3_EL1, SCTLRX, IMP))
>  
> -static inline bool kvm_arch_has_irq_bypass(void)
> +static inline bool kvm_arch_has_irq_bypass(struct kvm *kvm)
>  {
> -	return true;
> +	return vgic_supports_direct_msis(kvm);
>  }
>  
>  void kvm_idregs_enforce_dependencies(struct kvm *kvm);

[Severity: Medium]
Can kvm_irqfd_assign() skip irq_bypass_register_consumer() when userspace
creates the virtual ITS after initializing the VGICv3 distributor and
assigning irqfds?

kvm_irqfd_assign() only requires kvm_arch_intc_initialized(kvm)
(vgic_initialized(kvm)) and evaluates kvm_arch_has_irq_bypass(kvm) once
without holding kvm->arch.config_lock.  vgic_supports_direct_msis(kvm)
checks vgic_has_its(kvm), which tests dist->its_model, but vgic_its_create()
explicitly allows creating a KVM_DEV_TYPE_ARM_VGIC_ITS device after the
VGIC is already initialized:

arch/arm64/kvm/vgic/vgic-its.c:vgic_its_create() {
	...
	if (vgic_initialized(dev->kvm)) {
		ret = vgic_v4_init(dev->kvm);
	...
	dev->kvm->arch.vgic.msis_require_devid = true;
	dev->kvm->arch.vgic.its_model = KVM_DEV_TYPE_ARM_VGIC_V3;
}

If userspace initializes the VGICv3 distributor, assigns an irqfd via
KVM_IRQFD, and then creates the ITS and configures MSI routing,
vgic_supports_direct_msis(kvm) returns false during kvm_irqfd_assign().
Because vgic_its_create() does not register existing irqfds as irq_bypass
consumers, GICv4 direct vLPI forwarding is never enabled for those irqfds
and interrupts silently fall back to software injection.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261113093840.1034795-1-yangjinqian1@huawei.com?part=1