[PATCH v2 12/23] KVM: x86: Disable APIC logical map if logical ID covers multiple MDAs

Sean Christopherson
2022-09-03T00:23:57Z

Disable the optimized APIC logical map if a logical ID covers multiple
MDAs, i.e. if a vCPU has multiple bits set in its ID. In logical mode,
events match if "ID & MDA != 0", i.e. creating an entry for only the
first bit can cause interrupts to be missed.

Note, creating an entry for every bit is also wrong as KVM would generate
IPIs for every matching bit. It would be possible to teach KVM to play
nice with this edge case, but it is very much an edge case and probably
not used in any real world OS, i.e. it's not worth optimizing.

Use an impossible value for the "mode" to effectively designate that it's
disabled. Don't bother adding a dedicated "invalid" value, the mode
handling will be cleaned up in the future and it would take just as much
effort to explain what value is "safe" at this time.

Fixes: 1e08ec4a130e ("KVM: optimize apic interrupt delivery")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
---
arch/x86/kvm/lapic.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index d956cd37908e..6b2f538b8fd0 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -300,8 +300,15 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
if (!kvm_apic_map_get_logical_dest(new, ldr, &cluster, &mask))
continue;

- if (mask)
- cluster[ffs(mask) - 1] = apic;
+ if (!mask)
+ continue;
+
+ if (!is_power_of_2(mask)) {
+ new->mode = KVM_APIC_MODE_XAPIC_FLAT |
+ KVM_APIC_MODE_XAPIC_CLUSTER;
+ continue;
+ }
+ cluster[ffs(mask) - 1] = apic;
}
out:
old = rcu_dereference_protected(kvm->arch.apic_map,
--
2.37.2.789.g6183377224-goog