[PATCH v2 9/9] KVM: x86: Short-circuit all of kvm_apic_set_base() if MSR value is unchanged

[Sean Christopherson <seanjc@google.com>]

Do nothing in from kvm_apic_set_base() if the incoming MSR value is the
same as the current value, as 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.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/lapic.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 7b2342e40e4e..59a64b703aad 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2582,9 +2582,6 @@ static void __kvm_apic_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)
@@ -2632,6 +2629,10 @@ int kvm_apic_set_base(struct kvm_vcpu *vcpu, u64 value, bool host_initiated)
 {
 	enum lapic_mode old_mode = kvm_get_apic_mode(vcpu);
 	enum lapic_mode new_mode = kvm_apic_mode(value);
+
+	if (vcpu->arch.apic_base == value)
+		return 0;
+
 	u64 reserved_bits = kvm_vcpu_reserved_gpa_bits_raw(vcpu) | 0x2ff |
 		(guest_cpuid_has(vcpu, X86_FEATURE_X2APIC) ? 0 : X2APIC_ENABLE);
 
-- 
2.47.0.163.g1226f6d8fa-goog