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

[Huang, Kai]

On Tue, 2026-01-27 at 14:36 -0800, David Woodhouse wrote:
> 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.

To make it even more robust, perhaps we can grab kvm->lock mutex in
kvm_vm_ioctl_enable_cap() for KVM_CAP_X2APIC_API, so that it won't race with
KVM_CREATE_IRQCHIP (which already grabs kvm->lock) and
KVM_CAP_SPLIT_IRQCHIP?

Even more, we can add additional check in KVM_CREATE_IRQCHIP to return -
EINVAL when it sees kvm->arch.suppress_eoi_broadcast_mode is
KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST?