KVM: VMX: Skip pointless MSR bitmap update when setting EFER

Split setup_msrs() into vmx_setup_uret_msrs() and an open coded refresh
of the MSR bitmap, and skip the latter when refreshing the user return
MSRs during an EFER load.  Only the x2APIC MSRs are dynamically exposed
and hidden, and those are not affected by a change in EFER.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210713163324.627647-34-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -1647,11 +1647,12 @@ static void vmx_setup_uret_msr(struct vcpu_vmx *vmx, unsigned int msr,
 }
 
 /*
- * Set up the vmcs to automatically save and restore system
- * msrs.  Don't touch the 64-bit msrs if the guest is in legacy
- * mode, as fiddling with msrs is very expensive.
+ * Configuring user return MSRs to automatically save, load, and restore MSRs
+ * that need to be shoved into hardware when running the guest.  Note, omitting
+ * an MSR here does _NOT_ mean it's not emulated, only that it will not be
+ * loaded into hardware when running the guest.
  */
-static void setup_msrs(struct vcpu_vmx *vmx)
+static void vmx_setup_uret_msrs(struct vcpu_vmx *vmx)
 {
 #ifdef CONFIG_X86_64
 	bool load_syscall_msrs;
@@ -1681,9 +1682,6 @@ static void setup_msrs(struct vcpu_vmx *vmx)
 	 */
 	vmx_setup_uret_msr(vmx, MSR_IA32_TSX_CTRL, boot_cpu_has(X86_FEATURE_RTM));
 
-	if (cpu_has_vmx_msr_bitmap())
-		vmx_update_msr_bitmap(&vmx->vcpu);
-
 	/*
 	 * The set of MSRs to load may have changed, reload MSRs before the
 	 * next VM-Enter.
@@ -2874,7 +2872,7 @@ int vmx_set_efer(struct kvm_vcpu *vcpu, u64 efer)
 
 		msr->data = efer & ~EFER_LME;
 	}
-	setup_msrs(vmx);
+	vmx_setup_uret_msrs(vmx);
 	return 0;
 }
 
@@ -4470,7 +4468,10 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
 	if (kvm_mpx_supported())
 		vmcs_write64(GUEST_BNDCFGS, 0);
 
-	setup_msrs(vmx);
+	vmx_setup_uret_msrs(vmx);
+
+	if (cpu_has_vmx_msr_bitmap())
+		vmx_update_msr_bitmap(&vmx->vcpu);
 
 	vmcs_write32(VM_ENTRY_INTR_INFO_FIELD, 0);  /* 22.2.1 */