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

["Huang, Kai" <kai.huang@intel.com>]

On Tue, 2026-01-27 at 19:48 -0800, David Woodhouse wrote:
> On Wed, 2026-01-28 at 02:22 +0000, Huang, Kai wrote:
> >  
> > > 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?
> 
> If we do that, then the query for KVM_CAP_X2APIC_API could advertise
> the KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST for a freshly created KVM,
> even before userspace has enabled *either* KVM_CREATE_IRQCHIP nor
> KVM_CAP_SPLIT_IRQCHIP?

No IIUC it doesn't change that?

The change I mentioned above is only related to "enable" part, but not
"query" part.

The "query" is done via kvm_vm_ioctl_check_extension(KVM_CAP_X2APIC_API),
and in this patch, it does:

@@ -4931,6 +4933,8 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long
ext)
 		break;
 	case KVM_CAP_X2APIC_API:
 		r = KVM_X2APIC_API_VALID_FLAGS;
+		if (kvm && !irqchip_split(kvm))
+			r &= ~KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST;

IIRC if this is called before KVM_CREATE_IRQCHIP and KVM_CAP_SPLIT_IRQCHIP,
then !irqchip_split() will be true, so it will NOT advertise
KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST.

If it is called after KVM_CAP_SPLIT_IRQCHIP, then it will advertise
KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST.

Btw, it doesn't grab kvm->lock either, so theoretically it could race with
KVM_CREATE_IRQCHIP and kvm_vm_ioctl_enable_cap(KVM_CAP_SPLIT_IRQCHIP) too.

> 
> That would be slightly better than the existing proposed awfulness
> where the kernel doesn't *admit* to having the _ENABLE_ capability
> until userspace first enables the KVM_CAP_SPLIT_IRQCHIP.

We could also make kvm_vm_ioctl_check_extension(KVM_CAP_X2APIC_API) to
_always_ advertise KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST if that's
better.

I suppose what we need is to document such behaviour -- that albeit 
KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST is advertise as supposed, but it
cannot be enabled together with KVM_CREATE_IRQCHIP -- one will fail
depending on which is called first.

As a bonus, it can get rid of "calling irqchip_split() w/o holding kvm-
>lock" awfulness too.