think I'd even prefer here a simple
aid = kvm_xapic_id(apic);
aid = kvm_x2apic_id(apic);
that would keep changes minimal and I don't really see any benefit in the
code when splitting handling up.
It is neccesassary to write an entry for both IDs and I wanted to split
it before [4/4], because doing both changes at once seemed hard to
Putting it here didn't work well either ... is a separate patch for the
hunk below better, or would you prefer to have it in [4/4]?
Patch 4 then simply can fixup setting code
if (aid <= new->max_apic_id && !new->phys_map[aid])
new->phys_map[aid] = apic;
(if I am not missing some important corner case here)
The trick is that we want to do the following even in xAPIC mode:
new->phys_map[kvm_x2apic_id(apic)] = apic;
This is the main idea of the hotplug hack -- to allow unique addressing
of processors that were reset in xAPIC mode. (And I add a disgusting
"x2apic_id > 0xff" condition in [4/4], because we still allow guests to
change xAPIC IDs, which wouldn't play nice with this.)
Hardware does a superset of this, because it only looks at lower 8 bits
of the desination ID when delivering to xAPIC.
When kvm_x2apic_id(apic) != kvm_xapic_id(apic), then the APIC is in
xAPIC mode so we definitely want to keep xAPIC working, hence
new->phys_map[kvm_xapic_id(apic)] = apic;