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

[Sean Christopherson <seanjc@google.com>]

On Tue, Jan 27, 2026, 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?

Only if userspace creates a VM and uses that to check capabilities, in which case
KVM is 100% right to say that KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST isn't
supported.  If userspace checks the system-scoped ioctl, i.e. with @kvm=NULL, it
will see KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST.

> 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? 

Hmm, if we want to improve that particular documentation, then we should do so
in the description of KVM_CHECK_EXTENSION itself, which currently says: 

  Based on their initialization different VMs may have different capabilities.
  It is thus encouraged to use the vm ioctl to query for capabilities (available
  with KVM_CAP_CHECK_EXTENSION_VM on the vm fd)

Because there multiple capabilities that are conditionally supported based on
the VM type/configuration, i.e. this behavior isn't novel.

> 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.

Eh, VM really should be returning '0' for the check for all KVM_CAP_X2APIC_API,
and disallowing the capability, if the VM doesn't have an in-kernel local APIC.
Because enabling any of the KVM_X2APIC_API_* options without a local APIC doesn't
actually do anything.

I say that because I'd be very tempted to "fix" that by restricting new flags to
VMs with irqchip_in_kernel(), at which point userspace needs to get the ordering
right anyways.