linux-kernel - Re: [PATCH v6] KVM: x86: Add x2APIC "features" to control EOI broadcast suppression

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Message-ID: <70f67bfb787a8a01302c1e1b25b1d52e72320305.camel@infradead.org>
Date: Tue, 27 Jan 2026 21:32:14 -0800
From: David Woodhouse <dwmw2@...radead.org>
To: Khushit Shah <khushit.shah@...anix.com>, "Huang, Kai"
 <kai.huang@...el.com>,  "seanjc@...gle.com" <seanjc@...gle.com>
Cc: Shaju Abraham <shaju.abraham@...anix.com>, "x86@...nel.org"
 <x86@...nel.org>,  "bp@...en8.de" <bp@...en8.de>, "stable@...r.kernel.org"
 <stable@...r.kernel.org>,  "hpa@...or.com" <hpa@...or.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
 "mingo@...hat.com" <mingo@...hat.com>, "dave.hansen@...ux.intel.com"
 <dave.hansen@...ux.intel.com>,  "pbonzini@...hat.com"
 <pbonzini@...hat.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, Jon
 Kohler <jon@...anix.com>, "tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [PATCH v6] KVM: x86: Add x2APIC "features" to control EOI
 broadcast suppression

On Wed, 2026-01-28 at 05:17 +0000, Khushit Shah wrote:
> 
> How about we make an explicit _ENABLE_ bit for split IRQCHIP?
> When/if in-kernel IRQCHIP starts supporting I/O APIC 0x20, we
> can add a separate bit for that in the CAP. 
> 
> This way:
> - The flag name (KVM_X2APIC_SPLIT_ENABLE_SEOIB) is self-documenting.
> - We always advertise it in KVM_CHECK_EXTENSION.
> - Enabling requires split IRQCHIP to be configured first.
> - Mutex protects against races with KVM_CAP_SPLIT_IRQCHIP.

Ick. The more we iterate on this, the more convinced I am that we
should just enable it for I/O APIC at the same time. Userspace has to
explicitly opt in to the combination of kernel I/O APIC and SEOIB
anyway.

So I'll just bow out of the conversation; do whatever you think best.

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