linux-kernel - Re: [PATCH v5 1/3] KVM: x86: Refactor suppress EOI broadcast logic

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Message-ID: <E19BAC02-F4E4-4F66-A85D-A0D12D355E23@nutanix.com>
Date: Fri, 16 Jan 2026 10:02:03 +0000
From: Khushit Shah <khushit.shah@...anix.com>
To: David Woodhouse <dwmw2@...radead.org>
CC: Sean Christopherson <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 1/3] KVM: x86: Refactor suppress EOI broadcast logic



> On 16 Jan 2026, at 2:31 PM, David Woodhouse <dwmw2@...radead.org> wrote:
> 
> But KVM *will* notify listeners, surely? When the guest issues the EOI
> via the I/O APIC EOIR register.
> 
> For that commit to have made any difference, Xen *has* to have been
> buggy, enabling directed EOI in the local APIC despite the I/O APIC not
> having the required support. Thus interrupts never got EOI'd at all,
> and sure, the notifiers didn't get called.

You are describing 
0bcc3fb95b97 ("KVM: lapic: stop advertising DIRECTED_EOI when in-kernel IOAPIC is in use”)
Since then I guess this issue should have been fixed?!  As
c806a6ad35bf ("KVM: x86: call irq notifiers with directed EOI”)  was much earlier.

> On 16 Jan 2026, at 2:31 PM, David Woodhouse <dwmw2@...radead.org> wrote:
> 
> If you're concerned about what to backport to stable, then arguably
> it's *only* KVM_X2APIC_DISABLE_SUPPRESS_EOI_BROADCAST which should be
> backported, as that's the bug, and _ENABLE_ is a new feature?

I think neither DISABLE or ENABLE is a new feature at least for split IRQCHIP.
It’s just giving a way to user-space to fix a bug in a way they like, because that’s how
it should have been from the beginning.