KVM: MIPS: Update kvm_lose_fpu() for VZ

Update the implementation of kvm_lose_fpu() for VZ, where there is no
need to enable the FPU/MSA in the root context if the FPU/MSA state is
loaded but disabled in the guest context.

The trap & emulate implementation needs to disable FPU/MSA in the root
context when the guest disables them in order to catch the COP1 unusable
or MSA disabled exception when they're used and pass it on to the guest.

For VZ however as long as the context is loaded and enabled in the root
context, the guest can enable and disable it in the guest context
without the hypervisor having to do much, and will take guest exceptions
without hypervisor intervention if used without being enabled in the
guest context.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org

arch/mips/kvm/mips.c

@@ -1527,16 +1527,18 @@ void kvm_drop_fpu(struct kvm_vcpu *vcpu)
 void kvm_lose_fpu(struct kvm_vcpu *vcpu)
 {
 	/*
-	 * FPU & MSA get disabled in root context (hardware) when it is disabled
-	 * in guest context (software), but the register state in the hardware
-	 * may still be in use. This is why we explicitly re-enable the hardware
-	 * before saving.
+	 * With T&E, FPU & MSA get disabled in root context (hardware) when it
+	 * is disabled in guest context (software), but the register state in
+	 * the hardware may still be in use.
+	 * This is why we explicitly re-enable the hardware before saving.
 	 */
 
 	preempt_disable();
 	if (cpu_has_msa && vcpu->arch.aux_inuse & KVM_MIPS_AUX_MSA) {
-		set_c0_config5(MIPS_CONF5_MSAEN);
-		enable_fpu_hazard();
+		if (!IS_ENABLED(CONFIG_KVM_MIPS_VZ)) {
+			set_c0_config5(MIPS_CONF5_MSAEN);
+			enable_fpu_hazard();
+		}
 
 		__kvm_save_msa(&vcpu->arch);
 		trace_kvm_aux(vcpu, KVM_TRACE_AUX_SAVE, KVM_TRACE_AUX_FPU_MSA);
@@ -1549,8 +1551,10 @@ void kvm_lose_fpu(struct kvm_vcpu *vcpu)
 		}
 		vcpu->arch.aux_inuse &= ~(KVM_MIPS_AUX_FPU | KVM_MIPS_AUX_MSA);
 	} else if (vcpu->arch.aux_inuse & KVM_MIPS_AUX_FPU) {
-		set_c0_status(ST0_CU1);
-		enable_fpu_hazard();
+		if (!IS_ENABLED(CONFIG_KVM_MIPS_VZ)) {
+			set_c0_status(ST0_CU1);
+			enable_fpu_hazard();
+		}
 
 		__kvm_save_fpu(&vcpu->arch);
 		vcpu->arch.aux_inuse &= ~KVM_MIPS_AUX_FPU;