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: <D6CA802E-F7E0-410D-87FB-6E6E5897460E@infradead.org>
Date: Mon, 29 Dec 2025 15:36:00 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: Khushit Shah <khushit.shah@...anix.com>
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>
Subject: Re: [PATCH v5 2/3] KVM: x86/ioapic: Implement support for I/O APIC version 0x20 with EOIR

On 29 December 2025 15:16:40 GMT, Khushit Shah <khushit.shah@...anix.com> wrote:
>
>
>> On 29 Dec 2025, at 6:31 PM, David Woodhouse <dwmw2@...radead.org> 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.