Re: [PATCH v3] KVM: x86: Add x2APIC "features" to control EOI broadcast suppression

[Paolo Bonzini]

On Wed, Dec 3, 2025 at 2:32 PM David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
> > That would make it impossible to use the fixed implementation on the
> > local APIC side, without changing the way the IOAPIC appears to the
> > guest.
>
> Yes, but remember that "the fixed implementation on the local APIC
> side" means precisely that it's fixed to *not* broadcast the EOI. Which
> means you absolutely *need* to have an I/O APIC capable of receiving
> the explicit directed EOI, or the EOI will never happen at all.
>
> Which is why it probably makes sense to drop the 'version_id' field
> from the struct where I'd added it, and just make the code report a
> hard-coded version based on suppress_eoi_broadcast being enabled:
>
> (kvm->arch.suppress_eoi_broadcast == KVM_SUPPRESS_EOI_ENABLED) ? 0x20: 0x11
>
> So yes, it's a guest-visible change, but only if the VMM explicitly
> *asks* for the broadcast suppression feature to work, in which case
> it's *necessary* anyway.

I see what you mean and I guess you're right... "Setting X will cause
the in-kernel IOAPIC to report version 0x20" is as obscure as it gets,
but then so is "Setting X will break guests unless you tell in-kernel
IOAPIC to report version 0x20".

So this is good, but the docs need to say clearly that this should
only be set if either full in-kernel irqchip is in use or, for split
irqchip, if the userspace IOAPIC implements directed EOI correctly.

Paolo