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: <BE16B024-0BE6-46B4-A1B4-7B2F00E4107B@nutanix.com>
Date: Mon, 29 Dec 2025 15:16:40 +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>
Subject: Re: [PATCH v5 2/3] KVM: x86/ioapic: Implement support for I/O APIC
 version 0x20 with EOIR



> 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 ;)