Re: [PATCH v4] KVM: x86: Add x2APIC "features" to control EOI broadcast suppression

[David Woodhouse]

On Thu, 2025-12-11 at 10:59 +0000, Khushit Shah wrote:
> 
> +Modern VMMs should either enable KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST or
> +KVM_X2APIC_DISABLE_SUPPRESS_EOI_BROADCAST. If not, legacy quirky behavior will
> +be used by KVM, which is to advertise support for Suppress EOI Broadcasts but
> +not actually suppressing EOI broadcasts.

"which is to... not actually suppressing EOI broadcasts."

I think you want s/suppressing/suppress/ there.

> +
> +Currently, both KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST and


I don't much like the word 'both' at the start of the sentence. It sent
my brain down a language-recognition path which thought it was about
setting both bits at the same time, and may lead to misunderstandings
or just slower parsing of the sentence.

> +KVM_X2APIC_DISABLE_SUPPRESS_EOI_BROADCAST must only be set when in split IRQCHIP
> +mode. Otherwise, the ioctl will fail with an EINVAL error.

Hm, I don't like that much. For a start, DISABLE should be fine with
the in-kernel IRQCHIP right now (and is the only behaviour that truly
makes sense right now).

And my intent was that the in-kernel I/O APIC patch gets included as
*part* of this series, otherwise we're making a semantic change to the
ENABLE behaviour later.

Also... how does userspace discover the availability of these flags?

(And if you don't include the I/O APIC patch as part of this series, we
also need to understand how userspace will later discover that ENABLE
can be applied to the in-kernel irqchip too.)

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