intel_iommu: reject broken EIM

intel_iommu: reject broken EIM

Cluster x2APIC cannot work without KVM's x2apic API when the maximal
APIC ID is greater than 8 and only KVM's LAPIC can support x2APIC, so we
forbid other APICs and also the old KVM case with less than 9, to
simplify the code.

There is no point in enabling EIM in forbidden APICs, so we keep it
enabled only for the KVM APIC;  unconditionally, because making the
option depend on KVM version would be a maintanance burden.

Old QEMUs would enable eim whenever intremap was on, which would trick
guests into thinking that they can enable cluster x2APIC even if any
interrupt destination would get clamped to 8 bits.
Depending on your configuration, QEMU could notice that the destination
LAPIC is not present and report it with a very non-obvious:

  KVM: injection failed, MSI lost (Operation not permitted)

Or the guest could say something about unexpected interrupts, because
clamping leads to aliasing so interrupts were being delivered to
incorrect VCPUs.

KVM_X2APIC_API is the feature that allows us to enable EIM for KVM.

QEMU 2.7 allowed EIM whenever interrupt remapping was enabled.  In order
to keep backward compatibility, we again allow guests to misbehave in
non-obvious ways, and make it the default for old machine types.

A user can enable the buggy mode it with "x-buggy-eim=on".

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>