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

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Message-ID: <DD13B2B3-5719-410F-8B98-9DB3E1738997@nutanix.com>
Date: Mon, 29 Dec 2025 12:21:02 +0000
From: Khushit Shah <khushit.shah@...anix.com>
To: David Woodhouse <dwmw2@...radead.org>
CC: "seanjc@...gle.com" <seanjc@...gle.com>,
        "pbonzini@...hat.com"
	<pbonzini@...hat.com>,
        "kai.huang@...el.com" <kai.huang@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
        "hpa@...or.com" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        Jon
 Kohler <jon@...anix.com>,
        Shaju Abraham <shaju.abraham@...anix.com>,
        David
 Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH v5 2/3] KVM: x86/ioapic: Implement support for I/O APIC
 version 0x20 with EOIR


> On 29 Dec 2025, at 5:09 PM, David Woodhouse <dwmw2@...radead.org> wrote:
> 
> On 29 December 2025 11:17:07 GMT, Khushit Shah <khushit.shah@...anix.com> wrote:
>> From: David Woodhouse <dwmw@...zon.co.uk>
>> 
>> 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@...zon.co.uk>
>> Signed-off-by: Khushit Shah <khushit.shah@...anix.com>
>> ---
>> 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*.