KVM: x86: Short-circuit all kvm_lapic_set_base() if MSR value isn't changing

Do nothing in kvm_lapic_set_base() if the APIC base MSR value is the same
as the current value.  All flows except the handling of the base address
explicitly take effect if and only if relevant bits are changing.

For the base address, invoking kvm_lapic_set_base() before KVM initializes
the base to APIC_DEFAULT_PHYS_BASE during vCPU RESET would be a KVM bug,
i.e. KVM _must_ initialize apic->base_address before exposing the vCPU (to
userspace or KVM at-large).

Note, the inhibit is intended to be set if the base address is _changed_
from the default, i.e. is also covered by the RESET behavior.

Reviewed-by: Kai Huang <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

Author: sean-jc

arch/x86/kvm/lapic.c

@@ -2582,6 +2582,9 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value)
 	u64 old_value = vcpu->arch.apic_base;
 	struct kvm_lapic *apic = vcpu->arch.apic;
 
+	if (old_value == value)
+		return;
+
 	vcpu->arch.apic_base = value;
 
 	if ((old_value ^ value) & MSR_IA32_APICBASE_ENABLE)