Re: [PATCH v4 1/4] KVM: x86: ioapic: Update remote_irr only after successful delivery

["Huang, Kai" <kai.huang@intel.com>]

On Tue, 2026-08-11 at 15:34 +0800, Hao Zhang wrote:
> From: Hao Zhang <zhanghao1@kylinos.cn>
> 
> The I/O APIC sets remote_irr for level-triggered interrupts that have
> been accepted by a local APIC and are waiting for an EOI.
> 
> ioapic_service() currently treats any non-zero return from
> kvm_irq_delivery_to_apic() as successful delivery.  But
> kvm_irq_delivery_to_apic() returns -1 when no destination is found.
> Setting remote_irr for that failed delivery leaves the pin blocked
> forever waiting for an EOI that will never be generated.
> 
> Update remote_irr only when the delivery result is positive, i.e. when
> at least one local APIC accepted the interrupt.
> 
> Fixes: 4925663a079c ("KVM: Report IRQ injection status to userspace.")

The code LGTM, but I am not sure this is the right commit for the Fixes tag.

Looking at the relevant diff:

+static int ioapic_service(struct kvm_ioapic *ioapic, unsigned int idx)
 {
        union ioapic_redir_entry *pent;
+       int injected = -1;
 
        pent = &ioapic->redirtbl[idx];
 
        if (!pent->fields.mask) {
-               int injected = ioapic_deliver(ioapic, idx);
+               injected = ioapic_deliver(ioapic, idx);
                if (injected && pent->fields.trig_mode == IOAPIC_LEVEL_TRIG)
                        pent->fields.remote_irr = 1;
        }

This commit didn't change any logic related to remote_irr, since it didn't touch
ioapic_deliver() at all.

The one that is close seems to be 

  ff4b9df877b30 ("KVM: IOAPIC: only set remote_irr if interrupt was injected")

since it actually touched the logic of when to set remote_irr:

@@ -89,8 +89,8 @@ static void ioapic_service(struct kvm_ioapic *ioapic, unsigned
int idx)
        pent = &ioapic->redirtbl[idx];
 
        if (!pent->fields.mask) {
-               ioapic_deliver(ioapic, idx);
-               if (pent->fields.trig_mode == IOAPIC_LEVEL_TRIG)
+               int injected = ioapic_deliver(ioapic, idx);
+               if (injected && pent->fields.trig_mode == IOAPIC_LEVEL_TRIG)
                        pent->fields.remote_irr = 1;
        }

But looking closer, at that time the ioapic_deliver() could only return 0 or 1
(or >1 I didn't check), i.e., it couldn't return -1 at all.

In fact, even when commit 4925663a079c was merged the then ioapic_deliver()
couldn't return -1.

So AFAICT both of the above two commits are not the right one.

I haven't found which one to blame, but it seems at some point the function
kvm_irq_delivery_to_apic() was introduced and at some other point it was changed
to be able to return -1.  But I didn't dig further.