KVM: VMX: Skip emulation required checks during pmode/rmode transitions

Don't refresh "emulation required" when stuffing segments during
transitions to/from real mode when running without unrestricted guest.
The checks are unnecessary as vmx_set_cr0() unconditionally rechecks
"emulation required".  They also happen to be broken, as enter_pmode()
and enter_rmode() run with a stale vcpu->arch.cr0.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210713163324.627647-29-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -2719,6 +2719,8 @@ static __init int alloc_kvm_area(void)
 	return 0;
 }
 
+static void __vmx_set_segment(struct kvm_vcpu *vcpu, struct kvm_segment *var, int seg);
+
 static void fix_pmode_seg(struct kvm_vcpu *vcpu, int seg,
 		struct kvm_segment *save)
 {
@@ -2735,7 +2737,7 @@ static void fix_pmode_seg(struct kvm_vcpu *vcpu, int seg,
 		save->dpl = save->selector & SEGMENT_RPL_MASK;
 		save->s = 1;
 	}
-	vmx_set_segment(vcpu, save, seg);
+	__vmx_set_segment(vcpu, save, seg);
 }
 
 static void enter_pmode(struct kvm_vcpu *vcpu)
@@ -2756,7 +2758,7 @@ static void enter_pmode(struct kvm_vcpu *vcpu)
 
 	vmx->rmode.vm86_active = 0;
 
-	vmx_set_segment(vcpu, &vmx->rmode.segs[VCPU_SREG_TR], VCPU_SREG_TR);
+	__vmx_set_segment(vcpu, &vmx->rmode.segs[VCPU_SREG_TR], VCPU_SREG_TR);
 
 	flags = vmcs_readl(GUEST_RFLAGS);
 	flags &= RMODE_GUEST_OWNED_EFLAGS_BITS;
@@ -3291,7 +3293,7 @@ static u32 vmx_segment_access_rights(struct kvm_segment *var)
 	return ar;
 }
 
-void vmx_set_segment(struct kvm_vcpu *vcpu, struct kvm_segment *var, int seg)
+static void __vmx_set_segment(struct kvm_vcpu *vcpu, struct kvm_segment *var, int seg)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 	const struct kvm_vmx_segment_field *sf = &kvm_vmx_segment_fields[seg];
@@ -3304,7 +3306,7 @@ void vmx_set_segment(struct kvm_vcpu *vcpu, struct kvm_segment *var, int seg)
 			vmcs_write16(sf->selector, var->selector);
 		else if (var->s)
 			fix_rmode_seg(seg, &vmx->rmode.segs[seg]);
-		goto out;
+		return;
 	}
 
 	vmcs_writel(sf->base, var->base);
@@ -3326,9 +3328,13 @@ void vmx_set_segment(struct kvm_vcpu *vcpu, struct kvm_segment *var, int seg)
 		var->type |= 0x1; /* Accessed */
 
 	vmcs_write32(sf->ar_bytes, vmx_segment_access_rights(var));
+}
 
-out:
-	vmx->emulation_required = emulation_required(vcpu);
+void vmx_set_segment(struct kvm_vcpu *vcpu, struct kvm_segment *var, int seg)
+{
+	__vmx_set_segment(vcpu, var, seg);
+
+	to_vmx(vcpu)->emulation_required = emulation_required(vcpu);
 }
 
 static void vmx_get_cs_db_l_bits(struct kvm_vcpu *vcpu, int *db, int *l)