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

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Message-ID: <9CB80182-701E-4D28-A150-B3A0E774CD61@nutanix.com>
Date: Mon, 12 Jan 2026 04:15:37 +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 1/3] KVM: x86: Refactor suppress EOI broadcast logic



> On 2 Jan 2026, at 9:53 PM, David Woodhouse <dwmw2@...radead.org> wrote:
> 
> On Mon, 2025-12-29 at 11:17 +0000, Khushit Shah wrote:
>> Extract the suppress EOI broadcast (Directed EOI) logic into helper
>> functions and move the check from kvm_ioapic_update_eoi_one() to
>> kvm_ioapic_update_eoi() (required for a later patch). Prepare
>> kvm_ioapic_send_eoi() to honor Suppress EOI Broadcast in split IRQCHIP
>> mode.
>> 
>> Introduce two helper functions:
>> - kvm_lapic_advertise_suppress_eoi_broadcast(): determines whether KVM
>>   should advertise Suppress EOI Broadcast support to the guest
>> - kvm_lapic_respect_suppress_eoi_broadcast(): determines whether KVM should
>>   honor the guest's request to suppress EOI broadcasts
>> 
>> This refactoring prepares for I/O APIC version 0x20 support and userspace
>> control of suppress EOI broadcast behavior.
>> 
>> Signed-off-by: Khushit Shah <khushit.shah@...anix.com>
> 
> Looks good to me, thanks for pushing this through to completion!
> 
> 
> Reviewed-by: David Woodhouse <dwmw@...zon.co.uk>
> 
> Nit: Ideally I would would prefer to see an explicit 'no functional
> change intended' and a reference to commit 0bcc3fb95b97a.


I took another careful look at the refactor specifically through the
“no functional change” lens.

The legacy behavior with the in-kernel IRQCHIP can be summarized as:
- Suppress EOI Broadcast (SEOIB) is not advertised to the guest.
- If the guest nevertheless enables SEOIB, it is honored (already in un-s
  upported territory).
- Even in that case, the legacy code still ends up calling
  kvm_notify_acked_irq() in kvm_ioapic_update_eoi_one().

With the refactor, kvm_notify_acked_irq() is no longer reached in this
specific legacy scenario when the guest enables SEOIB despite it not
being advertised. I believe this is acceptable, as the guest is relying
on an unadvertised feature.

For non-QUIRKED configurations, the behavior is also correct:
- When SEOIB is ENABLED, kvm_notify_acked_irq() is called on EOIR write,
  when enabled by guest.
- When SEOIB is DISABLED, kvm_notify_acked_irq() is called on EOI
  broadcast.

I would appreciate others chiming in if they see a reason to preserve
the legacy ack behavior even in the unsupported case.