Re: [PATCH v2 2/4] KVM: x86: replace kvm_apic_id with kvm_{x,x2}apic_id

From: David Hildenbrand
Date: Wed Dec 14 2016 - 12:26:19 EST

think I'd even prefer here a simple

aid = kvm_xapic_id(apic);
if (apic_x2apic_mode(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]?

I actually would prefer to have it in 4/4, but not sure if it is worth yet another round. Anyhow,

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

for this patch (with the 256 fixed)

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

if (!apic_x2apic_mode(apic))
new->phys_map[kvm_xapic_id(apic)] = apic;

Okay, so this is is the case I missed in patch 4 :)

Thanks for the explanation and sorry for the noise.