KVM: x86: Short-circuit all of kvm_apic_set_base() if MSR value is unchanged

Do nothing in all of kvm_apic_set_base(), not just __kvm_apic_set_base(),
if the incoming MSR value is the same as the current value.  Validating
the mode transitions is obviously unnecessary, and rejecting the write is
pointless if the vCPU already has an invalid value, e.g. if userspace is
doing weird things and modified guest CPUID after setting MSR_IA32_APICBASE.

Bailing early avoids kvm_recalculate_apic_map()'s slow path in the rare
scenario where the map is DIRTY due to some other vCPU dirtying the map,
in which case it's the other vCPU/task's responsibility to recalculate the
map.

Note, kvm_lapic_reset() calls __kvm_apic_set_base() only when emulating
RESET, in which case the old value is guaranteed to be zero, and the new
value is guaranteed to be non-zero.  I.e. all callers of
__kvm_apic_set_base() effectively pre-check for the MSR value actually
changing.  Don't bother keeping the check in __kvm_apic_set_base(), as no
additional callers are expected, and implying that the MSR might already
be non-zero at the time of kvm_lapic_reset() could confuse readers.

Link: https://lore.kernel.org/r/20241101183555.1794700-10-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>