Re: [PATCH v5 2/3] KVM: x86/ioapic: Implement support for I/O APIC version 0x20 with EOIR

[David Woodhouse]

On 29 December 2025 15:16:40 GMT, Khushit Shah <khushit.shah@xxxxxxxxxxx> wrote:
>
>
>> On 29 Dec 2025, at 6:31 PM, David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
>> 
>> Hm? IIUC kvm_lapic_advertise_suppress_eoi_broadcast() is true whenever
>> userspace *hasn't* set KVM_X2APIC_DISABLE_SUPPRESS_EOI_BROADCAST
>> (either userspace has explicitly *enabled* it instead, or userspace has
>> done neither and we should preserve the legacy behaviour).
>
>The legacy behaviour for "kvm_lapic_advertise_suppress_eoi_broadcast()" is:
>- true for split IRQCHIP (userspace I/O APIC)
>- false for in-kernel IRQCHIP
>
>The in-kernel IRQCHIP case was "fixed" by commit 0bcc3fb95b97 ("KVM: lapic:
>stop advertising DIRECTED_EOI when in-kernel IOAPIC is in use"), which made
>it return false when IOAPIC is in-kernel.
>
>With this series, in QUIRKED mode the function still returns !ioapic_in_kernel(),
>preserving that exact legacy behavior. The I/O APIC version 0x20 (with EOIR)
>is only used when userspace explicitly sets the ENABLE flag.
>
>The comments in patch 1 explain this in more detail ;)

Ah, OK. So in the case of in-kernel I/O APIC, kvm_lapic_advertise_suppress_eoi_broadcast() kvm_lapic_respect_suppress_eoi_broadcast() are the same. In that case we can choose the one which is easier to understand and doesn't need the reader to refer back to an earlier commit? I accept your correction; the patch is correct.

But I think I still prefer the check to be on _respect_ as it's clearer that it's part of the new behaviour that is only introduced with this series.