Re: [PATCH] KVM: x86: don't hold kvm->lock in KVM_SET_GSI_ROUTING

["Radim Krčmář" <rkrcmar@redhat.com>]

2017-04-25 21:03+0200, David Hildenbrand:
> We needed the lock to avoid racing with creation of the irqchip on x86. As
> kvm_set_irq_routing() calls srcu_synchronize_expedited(), this lock
> might be held for a longer time.
> 
> Let's introduce an arch specific callback to check if we can actually
> add irq routes. For x86, all we have to do is check if we have an
> irqchip in the kernel. We don't need kvm->lock at that point as the
> irqchip is marked as inititalized only when actually fully created.
> 
> Reported-by: Steve Rutherford <srutherford@google.com>
> Fixes: 1df6ddede10a ("KVM: x86: race between KVM_SET_GSI_ROUTING and KVM_CREATE_IRQCHIP")
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  6 files changed, 18 insertions(+), 21 deletions(-)

Nice!

Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>

> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> @@ -504,6 +504,7 @@ void vcpu_put(struct kvm_vcpu *vcpu);
>  #ifdef __KVM_HAVE_IOAPIC
>  void kvm_arch_post_irq_ack_notifier_list_update(struct kvm *kvm);
>  void kvm_arch_post_irq_routing_update(struct kvm *kvm);
> +bool kvm_arch_can_set_irq_routing(struct kvm *kvm);

(A nitpick: it might be useful even without __KVM_HAVE_IOAPIC so weak
 linking would probably be cleaner for a slow path.)

>  #else
>  static inline void kvm_arch_post_irq_ack_notifier_list_update(struct kvm *kvm)
>  {
> @@ -511,6 +512,10 @@ static inline void kvm_arch_post_irq_ack_notifier_list_update(struct kvm *kvm)
>  static inline void kvm_arch_post_irq_routing_update(struct kvm *kvm)
>  {
>  }
> +static bool kvm_arch_can_set_irq_routing(struct kvm *kvm)
> +{
> +	return true;
> +}
>  #endif
>