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

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Message-ID: <ea294969d05fc9c37e72053d7343e11fa9ffdded.camel@infradead.org>
Date: Tue, 27 Jan 2026 14:36:57 -0800
From: David Woodhouse <dwmw2@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Khushit Shah <khushit.shah@...anix.com>, pbonzini@...hat.com, 
 kai.huang@...el.com, mingo@...hat.com, x86@...nel.org, bp@...en8.de,
 hpa@...or.com,  linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 dave.hansen@...ux.intel.com,  tglx@...utronix.de, jon@...anix.com,
 shaju.abraham@...anix.com,  stable@...r.kernel.org
Subject: Re: [PATCH v6] KVM: x86: Add x2APIC "features" to control EOI
 broadcast suppression

On Tue, 2026-01-27 at 13:49 -0800, Sean Christopherson wrote:
> 
> Nope, we should be good on that front, kvm->arch.irqchip_mode can't be changed
> once its set.  I.e. the irqchip_split() check could get a false negative if it's
> racing with KVM_CREATE_IRQCHIP, but it can't get a false positive and thus
> incorrectly allow KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST.

Ah, so userspace which checks all the kernel's capabilities *first*
will not see KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST advertised,
because it needs to enable KVM_CAP_SPLIT_IRQCHIP first?

I guess that's tolerable¹ but the documentation could make it clearer,
perhaps? I can see VMMs silently failing to detect the feature because
they just don't set split-irqchip before checking for it? 

¹ although I still kind of hate it and would have preferred to have the
  I/O APIC patch; userspace still has to intentionally *enable* that
  combination. But OK, I've reluctantly conceded that.

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