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

[Dmytro Maluka <dmy@semihalf.com>]

Hi Eric,

On 8/9/22 10:43 PM, Eric Auger wrote:
> 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@semihalf.com>
>> Link: https://lore.kernel.org/lkml/c7b7860e-ae3a-7b98-e97e-28a62470c470@semihalf.com/
>> ---
>>  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?

That would be quite different: it would be "masked on at least one of
both", while I want "masked on both (if both are used)".

> 
>> +	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()

Actually I think it would still be ok to call it after
kvm_register_irq_ack_notifier(), not necessarily before. We could then
miss a mask notification between kvm_register_irq_ack_notifier() and
kvm_register_and_fire_irq_mask_notifier(), but it's ok since
kvm_register_and_fire_irq_mask_notifier() would then immediately send a
new notification with the up-to-date mask value.

> 
>> + *
>> + * 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).

Ok.

> 
> 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)
>>  {
>