Re: [PATCH v2 2/5] KVM: x86: Add kvm_register_and_fire_irq_mask_notifier()

[Eric Auger]

Hi Dmytro,

On 8/5/22 21:39, Dmytro Maluka wrote:
> In order to implement postponing resamplefd notification until an
> interrupt is unmasked, we need not only to track changes of the
> interrupt mask state (which is already possible with
> kvm_register_irq_mask_notifier()) but also to know its initial
> mask state before any mask notifier has fired.
>
> Moreover, we need to do this initial check of the IRQ mask state in a
> race-free way, to ensure that we will not miss any further mask or
> unmask events after we check the initial mask state.
>
> So implement kvm_register_and_fire_irq_mask_notifier() which atomically
> registers an IRQ mask notifier and calls it with the current mask value
> of the IRQ. It does that using the same locking order as when calling
> notifier normally via kvm_fire_mask_notifiers(), to prevent deadlocks.
>
> Its implementation needs to be arch-specific since it relies on
> arch-specific synchronization (e.g. ioapic->lock and pic->lock on x86,
> or a per-IRQ lock on ARM vGIC) for serializing our initial reading of
> the IRQ mask state with a pending change of this mask state.
>
> For now implement it for x86 only, and for other archs add a weak dummy
> implementation which doesn't really call the notifier (as other archs
> don't currently implement calling notifiers normally via
> kvm_fire_mask_notifiers() either, i.e. registering mask notifiers has no
> effect on those archs anyway).
>
> Signed-off-by: Dmytro Maluka <dmy@xxxxxxxxxxxx>
> Link: https://lore.kernel.org/lkml/c7b7860e-ae3a-7b98-e97e-28a62470c470@xxxxxxxxxxxx/
> ---
>  arch/x86/include/asm/kvm_host.h |  1 +
>  arch/x86/kvm/i8259.c            |  6 ++++
>  arch/x86/kvm/ioapic.c           |  6 ++++
>  arch/x86/kvm/ioapic.h           |  1 +
>  arch/x86/kvm/irq_comm.c         | 57 +++++++++++++++++++++++++++++++++
>  include/linux/kvm_host.h        |  4 +++
>  virt/kvm/eventfd.c              | 31 ++++++++++++++++--
>  7 files changed, 104 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index dc76617f11c1..cf0571ed2968 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1834,6 +1834,7 @@ static inline int __kvm_irq_line_state(unsigned long *irq_state,
>  
>  int kvm_pic_set_irq(struct kvm_pic *pic, int irq, int irq_source_id, int level);
>  void kvm_pic_clear_all(struct kvm_pic *pic, int irq_source_id);
> +bool kvm_pic_irq_is_masked(struct kvm_pic *s, int irq);
>  
>  void kvm_inject_nmi(struct kvm_vcpu *vcpu);
>  
> diff --git a/arch/x86/kvm/i8259.c b/arch/x86/kvm/i8259.c
> index e1bb6218bb96..1eb3127f6047 100644
> --- a/arch/x86/kvm/i8259.c
> +++ b/arch/x86/kvm/i8259.c
> @@ -211,6 +211,12 @@ void kvm_pic_clear_all(struct kvm_pic *s, int irq_source_id)
>  	pic_unlock(s);
>  }
>  
> +/* Called with s->lock held. */
> +bool kvm_pic_irq_is_masked(struct kvm_pic *s, int irq)
> +{
> +	return !!(s->pics[irq >> 3].imr & (1 << irq));
> +}
> +
>  /*
>   * acknowledge interrupt 'irq'
>   */
> diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c
> index 765943d7cfa5..fab11de1f885 100644
> --- a/arch/x86/kvm/ioapic.c
> +++ b/arch/x86/kvm/ioapic.c
> @@ -478,6 +478,12 @@ void kvm_ioapic_clear_all(struct kvm_ioapic *ioapic, int irq_source_id)
>  	spin_unlock(&ioapic->lock);
>  }
>  
> +/* Called with ioapic->lock held. */
> +bool kvm_ioapic_irq_is_masked(struct kvm_ioapic *ioapic, int irq)
> +{
> +	return !!ioapic->redirtbl[irq].fields.mask;
> +}
> +
>  static void kvm_ioapic_eoi_inject_work(struct work_struct *work)
>  {
>  	int i;
> diff --git a/arch/x86/kvm/ioapic.h b/arch/x86/kvm/ioapic.h
> index 539333ac4b38..fe1f51319992 100644
> --- a/arch/x86/kvm/ioapic.h
> +++ b/arch/x86/kvm/ioapic.h
> @@ -114,6 +114,7 @@ void kvm_ioapic_destroy(struct kvm *kvm);
>  int kvm_ioapic_set_irq(struct kvm_ioapic *ioapic, int irq, int irq_source_id,
>  		       int level, bool line_status);
>  void kvm_ioapic_clear_all(struct kvm_ioapic *ioapic, int irq_source_id);
> +bool kvm_ioapic_irq_is_masked(struct kvm_ioapic *ioapic, int irq);
>  void kvm_get_ioapic(struct kvm *kvm, struct kvm_ioapic_state *state);
>  void kvm_set_ioapic(struct kvm *kvm, struct kvm_ioapic_state *state);
>  void kvm_ioapic_scan_entry(struct kvm_vcpu *vcpu,
> diff --git a/arch/x86/kvm/irq_comm.c b/arch/x86/kvm/irq_comm.c
> index f27e4c9c403e..4bd4218821a2 100644
> --- a/arch/x86/kvm/irq_comm.c
> +++ b/arch/x86/kvm/irq_comm.c
> @@ -234,6 +234,63 @@ void kvm_free_irq_source_id(struct kvm *kvm, int irq_source_id)
>  	mutex_unlock(&kvm->irq_lock);
>  }
>  
> +void kvm_register_and_fire_irq_mask_notifier(struct kvm *kvm, int irq,
> +					     struct kvm_irq_mask_notifier *kimn)
> +{
> +	struct kvm_pic *pic = kvm->arch.vpic;
> +	struct kvm_ioapic *ioapic = kvm->arch.vioapic;
> +	struct kvm_kernel_irq_routing_entry entries[KVM_NR_IRQCHIPS];
> +	struct kvm_kernel_irq_routing_entry *pic_e = NULL, *ioapic_e = NULL;
> +	int idx, i, n;
> +	bool masked;
> +
> +	mutex_lock(&kvm->irq_lock);
> +
> +	/*
> +	 * Not possible to detect if the guest uses the PIC or the
> +	 * IOAPIC. So assume the interrupt to be unmasked iff it is
> +	 * unmasked in at least one of both.
> +	 */
> +	idx = srcu_read_lock(&kvm->irq_srcu);
> +	n = kvm_irq_map_gsi(kvm, entries, irq);
> +	srcu_read_unlock(&kvm->irq_srcu, idx);
> +
> +	for (i = 0; i < n; i++) {
> +		if (entries[i].type != KVM_IRQ_ROUTING_IRQCHIP)
> +			continue;
> +
> +		switch (entries[i].irqchip.irqchip) {
> +		case KVM_IRQCHIP_PIC_MASTER:
> +		case KVM_IRQCHIP_PIC_SLAVE:
> +			pic_e = &entries[i];
> +			break;
> +		case KVM_IRQCHIP_IOAPIC:
> +			ioapic_e = &entries[i];
> +			break;
> +		default:
> +			break;
> +		}
> +	}
> +
> +	if (pic_e)
> +		spin_lock(&pic->lock);
> +	if (ioapic_e)
> +		spin_lock(&ioapic->lock);
> +
> +	__kvm_register_irq_mask_notifier(kvm, irq, kimn);
> +
> +	masked = (!pic_e || kvm_pic_irq_is_masked(pic, pic_e->irqchip.pin)) &&
> +		 (!ioapic_e || kvm_ioapic_irq_is_masked(ioapic, ioapic_e->irqchip.pin));
Looks a bit cryptic to me. Don't you want pic_e && masked on pic ||
ioapic_e && masked on ioapic?

> +	kimn->func(kimn, masked);
> +
> +	if (ioapic_e)
> +		spin_unlock(&ioapic->lock);
> +	if (pic_e)
> +		spin_unlock(&pic->lock);
> +
> +	mutex_unlock(&kvm->irq_lock);
> +}
> +
>  bool kvm_arch_can_set_irq_routing(struct kvm *kvm)
>  {
>  	return irqchip_in_kernel(kvm);
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index dd5f14e31996..55233eb18eb4 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1608,8 +1608,12 @@ void kvm_register_irq_ack_notifier(struct kvm *kvm,
>  				   struct kvm_irq_ack_notifier *kian);
>  void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
>  				   struct kvm_irq_ack_notifier *kian);
> +void __kvm_register_irq_mask_notifier(struct kvm *kvm, int irq,
> +				      struct kvm_irq_mask_notifier *kimn);
>  void kvm_register_irq_mask_notifier(struct kvm *kvm, int irq,
>  				    struct kvm_irq_mask_notifier *kimn);
> +void kvm_register_and_fire_irq_mask_notifier(struct kvm *kvm, int irq,
> +					     struct kvm_irq_mask_notifier *kimn);
>  void kvm_unregister_irq_mask_notifier(struct kvm *kvm, int irq,
>  				      struct kvm_irq_mask_notifier *kimn);
>  void kvm_fire_mask_notifiers(struct kvm *kvm, unsigned irqchip, unsigned pin,
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index 39403d9fbdcc..3007d956b626 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -519,15 +519,42 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
>  	kvm_arch_post_irq_ack_notifier_list_update(kvm);
>  }
>  
> +void __kvm_register_irq_mask_notifier(struct kvm *kvm, int irq,
> +				      struct kvm_irq_mask_notifier *kimn)
> +{
> +	kimn->irq = irq;
> +	hlist_add_head_rcu(&kimn->link, &kvm->irq_mask_notifier_list);
> +}
> +
>  void kvm_register_irq_mask_notifier(struct kvm *kvm, int irq,
>  				    struct kvm_irq_mask_notifier *kimn)
>  {
>  	mutex_lock(&kvm->irq_lock);
> -	kimn->irq = irq;
> -	hlist_add_head_rcu(&kimn->link, &kvm->irq_mask_notifier_list);
> +	__kvm_register_irq_mask_notifier(kvm, irq, kimn);
>  	mutex_unlock(&kvm->irq_lock);
>  }
>  
> +/*
> + * kvm_register_and_fire_irq_mask_notifier() registers the notifier and
> + * immediately calls it with the current mask value of the IRQ. It does
> + * that atomically, so that we will find out the initial mask state of
> + * the IRQ and will not miss any further mask or unmask events. It does
> + * that using the same locking order as when calling notifier normally
> + * via kvm_fire_mask_notifiers(), to prevent deadlocks.
you may document somewhere that it must be called before

kvm_register_irq_ack_notifier()

> + *
> + * Implementation is arch-specific since it relies on arch-specific
> + * (irqchip-specific) synchronization. Below is a weak dummy
> + * implementation for archs not implementing it yet, as those archs
> + * don't implement calling notifiers normally via
> + * kvm_fire_mask_notifiers() either, i.e. registering mask notifiers
> + * has no effect on those archs anyway.
I would advise you to put Marc in the loop for the whole series (adding
him in CC).

Thanks

Eric
> + */
> +void __weak kvm_register_and_fire_irq_mask_notifier(struct kvm *kvm, int irq,
> +						    struct kvm_irq_mask_notifier *kimn)
> +{
> +	kvm_register_irq_mask_notifier(kvm, irq, kimn);
> +}
> +
>  void kvm_unregister_irq_mask_notifier(struct kvm *kvm, int irq,
>  				      struct kvm_irq_mask_notifier *kimn)
>  {