[PATCH 0/3] KVM: arm64: nv: Fixes for Nested Virtualization issues

[Ganapatrao Kulkarni]

On 11-01-2023 03:24 am, Marc Zyngier wrote:
> On Tue, 10 Jan 2023 12:17:20 +0000,
> Ganapatrao Kulkarni <gankulkarni at os.amperecomputing.com> wrote:
>>
>> I am currently working around this with "nohlt" kernel param to
>> NestedVM. Any suggestions to handle/fix this case/issue and avoid the
>> slowness of booting of NestedVM with more cores?
>>
>> Note: Guest-Hypervisor and NestedVM are using default kernel installed
>> using Fedora 36 iso.
> 
> Despite what I said earlier, I have a vague idea here, thanks to the
> interesting call traces that you provided (this is really awesome work
> BTW, given how hard it is to trace things across 3 different kernels).
> 
> We can slightly limit the impact of the prepare/finish sequence if the
> guest hypervisor only accesses the active registers for SGIs/PPIs on
> the vcpu that owns them, forbidding any cross-CPU-to-redistributor
> access.
> 
> Something along these lines, which is only boot-tested. Let me know
> how this fares for you.
> 
> Thanks,
> 
> 	M.
> 
> diff --git a/arch/arm64/kvm/vgic/vgic-mmio.c b/arch/arm64/kvm/vgic/vgic-mmio.c
> index b32d434c1d4a..1cca45be5335 100644
> --- a/arch/arm64/kvm/vgic/vgic-mmio.c
> +++ b/arch/arm64/kvm/vgic/vgic-mmio.c
> @@ -473,9 +473,10 @@ int vgic_uaccess_write_cpending(struct kvm_vcpu *vcpu,
>    * active state can be overwritten when the VCPU's state is synced coming back
>    * from the guest.
>    *
> - * For shared interrupts as well as GICv3 private interrupts, we have to
> - * stop all the VCPUs because interrupts can be migrated while we don't hold
> - * the IRQ locks and we don't want to be chasing moving targets.
> + * For shared interrupts as well as GICv3 private interrupts accessed from the
> + * non-owning CPU, we have to stop all the VCPUs because interrupts can be
> + * migrated while we don't hold the IRQ locks and we don't want to be chasing
> + * moving targets.
>    *
>    * For GICv2 private interrupts we don't have to do anything because
>    * userspace accesses to the VGIC state already require all VCPUs to be
> @@ -484,7 +485,8 @@ int vgic_uaccess_write_cpending(struct kvm_vcpu *vcpu,
>    */
>   static void vgic_access_active_prepare(struct kvm_vcpu *vcpu, u32 intid)
>   {
> -	if (vcpu->kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3 ||
> +	if ((vcpu->kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3 &&
> +	     vcpu == kvm_get_running_vcpu()) ||

Thanks Marc for the patch!

I think, you mean not equal to?
+           vcpu != kvm_get_running_vcpu()) ||

With the change to not-equal, the issue is fixed and I could see the 
NestedVM booting is pretty fast with higher number of cores as well.

>   	    intid >= VGIC_NR_PRIVATE_IRQS)
>   		kvm_arm_halt_guest(vcpu->kvm);
>   }
> @@ -492,7 +494,8 @@ static void vgic_access_active_prepare(struct kvm_vcpu *vcpu, u32 intid)
>   /* See vgic_access_active_prepare */
>   static void vgic_access_active_finish(struct kvm_vcpu *vcpu, u32 intid)
>   {
> -	if (vcpu->kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3 ||
> +	if ((vcpu->kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3 &&
> +	     vcpu == kvm_get_running_vcpu()) ||

Same, not equal to.
>   	    intid >= VGIC_NR_PRIVATE_IRQS)
>   		kvm_arm_resume_guest(vcpu->kvm);
>   }
> 


Thanks,
Ganapat