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

[Khushit Shah]

> On 29 Dec 2025, at 5:09 PM, David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
> 
> On 29 December 2025 11:17:07 GMT, Khushit Shah <khushit.shah@xxxxxxxxxxx> wrote:
>> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>> 
>> Introduce support for I/O APIC version 0x20, which includes the EOI
>> Register (EOIR) for directed EOI.  The EOI register allows guests to
>> perform EOIs to individual I/O APICs instead of relying on broadcast EOIs
>> from the local APIC.
>> 
>> When Suppress EOI Broadcast (SEOIB) capability is advertised to the guest,
>> guests that enable it will EOI individual I/O APICs by writing to their
>> EOI register instead of relying on broadcast EOIs from the LAPIC.  Hence,
>> when SEOIB is advertised (so that guests can use it if they choose), use
>> I/O APIC version 0x20 to provide the EOI register.  This prepares for a
>> userspace API that will allow explicit control of SEOIB support, providing
>> a consistent interface for both in-kernel and split IRQCHIP mode.
>> 
>> Add a tracepoint (kvm_ioapic_directed_eoi) to track directed EOIs for
>> debugging and observability.
>> 
>> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
>> Signed-off-by: Khushit Shah <khushit.shah@xxxxxxxxxxx>
>> ---
>> arch/x86/kvm/ioapic.c | 31 +++++++++++++++++++++++++++++--
>> arch/x86/kvm/ioapic.h | 19 +++++++++++--------
>> arch/x86/kvm/trace.h  | 17 +++++++++++++++++
>> 3 files changed, 57 insertions(+), 10 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c
>> index 6bf8d110aece..eea1eb7845c4 100644
>> --- a/arch/x86/kvm/ioapic.c
>> +++ b/arch/x86/kvm/ioapic.c
>> @@ -48,8 +48,11 @@ static unsigned long ioapic_read_indirect(struct kvm_ioapic *ioapic)
>> 
>> switch (ioapic->ioregsel) {
>> case IOAPIC_REG_VERSION:
>> - result = ((((IOAPIC_NUM_PINS - 1) & 0xff) << 16)
>> -   | (IOAPIC_VERSION_ID & 0xff));
>> + if (kvm_lapic_advertise_suppress_eoi_broadcast(ioapic->kvm))
>> + result = IOAPIC_VERSION_ID_EOIR;
>> + else
>> + result = IOAPIC_VERSION_ID;
>> + result |= ((IOAPIC_NUM_PINS - 1) & 0xff) << 16;
> 
> I think that wants to depend on _respect_ not _advertise_? Otherwise you're changing existing behaviour in the legacy/quirk case where the VMM neither explicitly enables not disables the feature.

I think _advertise_ is correct, as for legacy case, in kernel IRQCHIP mode, _advertise_ is false. For kernel IRQCHIP, _advertise_ is only true when *enabled*.