Subject: Re: [PATCH v2] KVM: arm: VGIC: properly initialise private IRQ affinity

From: Marc Zyngier <[email-masked]>
Date: Thu, 22 Aug 2019 20:29:54 +0100

On Thu, 22 Aug 2019 18:05:10 +0100
Andre Przywara <[email-masked]> wrote:

> At the moment we initialise the target *mask* of a virtual IRQ to the
> VCPU it belongs to, even though this mask is only defined for GICv2 and
> quickly runs out of bits for many GICv3 guests.
> This behaviour triggers an UBSAN complaint for more than 32 VCPUs:
> ------
> [ 5659.462377] UBSAN: Undefined behaviour in virt/kvm/arm/vgic/vgic-init.c:223:21
> [ 5659.471689] shift exponent 32 is too large for 32-bit type 'unsigned int'
> ------
> Also for GICv3 guests the reporting of TARGET in the "vgic-state" debugfs
> dump is wrong, due to this very same problem.
> 
> Because there is no requirement to create the VGIC device before the
> VCPUs (and QEMU actually does it the other way round), we can't safely
> initialise mpidr or targets in kvm_vgic_vcpu_init(). But since we touch
> every private IRQ for each VCPU anyway later (in vgic_init()), we can
> just move the initialisation of those fields into there, where we
> definitely know the VGIC type.
> 
> On the way make sure we really have either a VGICv2 or a VGICv3 device,
> since the former checks was just checking for "VGICv3 or not", silently
> ignoring the uninitialised case.
> 
> Signed-off-by: Andre Przywara <[email-masked]>
> Reported-by: Dave Martin <[email-masked]>
> ---
> Hi,
> 
> tested with 4, 8 and 33 VCPUs with kvmtool and QEMU, on a GICv2 and a
> GICv3 machine.
> Also briefly tested localhost migration on the GICv3 machine w/ 33
> VCPUs, although I think all IRQs are group 1.
> 
> Cheers,
> Andre
> 
>  virt/kvm/arm/vgic/vgic-init.c | 28 ++++++++++++++++++----------
>  1 file changed, 18 insertions(+), 10 deletions(-)
> 
> diff --git a/virt/kvm/arm/vgic/vgic-init.c b/virt/kvm/arm/vgic/vgic-init.c
> index 80127ca9269f..413fb6a5525c 100644
> --- a/virt/kvm/arm/vgic/vgic-init.c
> +++ b/virt/kvm/arm/vgic/vgic-init.c
> @@ -8,6 +8,7 @@
>  #include <linux/cpu.h>
>  #include <linux/kvm_host.h>
>  #include <kvm/arm_vgic.h>
> +#include <asm/kvm_emulate.h>
>  #include <asm/kvm_mmu.h>
>  #include "vgic.h"
>  
> @@ -165,12 +166,17 @@ static int kvm_vgic_dist_init(struct kvm *kvm, unsigned int nr_spis)
>  		irq->vcpu = NULL;
>  		irq->target_vcpu = vcpu0;
>  		kref_init(&irq->refcount);
> -		if (dist->vgic_model == KVM_DEV_TYPE_ARM_VGIC_V2) {
> +		switch (dist->vgic_model) {
> +		case KVM_DEV_TYPE_ARM_VGIC_V2:
>  			irq->targets = 0;
>  			irq->group = 0;
> -		} else {
> +			break;
> +		case KVM_DEV_TYPE_ARM_VGIC_V3:
>  			irq->mpidr = 0;
>  			irq->group = 1;
> +			break;
> +		default:
> +			BUG_ON(1);
>  		}
>  	}
>  	return 0;
> @@ -210,7 +216,6 @@ int kvm_vgic_vcpu_init(struct kvm_vcpu *vcpu)
>  		irq->intid = i;
>  		irq->vcpu = NULL;
>  		irq->target_vcpu = vcpu;
> -		irq->targets = 1U << vcpu->vcpu_id;
>  		kref_init(&irq->refcount);
>  		if (vgic_irq_is_sgi(i)) {
>  			/* SGIs */
> @@ -220,11 +225,6 @@ int kvm_vgic_vcpu_init(struct kvm_vcpu *vcpu)
>  			/* PPIs */
>  			irq->config = VGIC_CONFIG_LEVEL;
>  		}
> -
> -		if (dist->vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3)
> -			irq->group = 1;
> -		else
> -			irq->group = 0;
>  	}
>  
>  	if (!irqchip_in_kernel(vcpu->kvm))
> @@ -287,10 +287,18 @@ int vgic_init(struct kvm *kvm)
>  
>  		for (i = 0; i < VGIC_NR_PRIVATE_IRQS; i++) {
>  			struct vgic_irq *irq = &vgic_cpu->private_irqs[i];
> -			if (dist->vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3)
> +			switch (dist->vgic_model) {
> +			case KVM_DEV_TYPE_ARM_VGIC_V3:
>  				irq->group = 1;
> -			else
> +				irq->mpidr = kvm_vcpu_get_mpidr_aff(vcpu);
> +				break;
> +			case KVM_DEV_TYPE_ARM_VGIC_V2:
>  				irq->group = 0;
> +				irq->targets = 1U << idx;
> +				break;
> +			default:
> +				BUG_ON(1);
> +			}
>  		}
>  	}
>  

Please drop the BUG_ON()s. If something is unexpected, just fail to
init the guest, but don't kill the box.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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