KVM: xen: do not use struct gfn_to_hva_cache

gfn_to_hva_cache is not thread-safe, so it is usually used only within
a vCPU (whose code is protected by vcpu->mutex).  The Xen interface
implementation has such a cache in kvm->arch, but it is not really
used except to store the location of the shared info page.  Replace
shinfo_set and shinfo_cache with just the value that is passed via
KVM_XEN_ATTR_TYPE_SHARED_INFO; the only complication is that the
initialization value is not zero anymore and therefore kvm_xen_init_vm
needs to be introduced.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -1003,9 +1003,8 @@ struct msr_bitmap_range {
 /* Xen emulation context */
 struct kvm_xen {
 	bool long_mode;
-	bool shinfo_set;
 	u8 upcall_vector;
-	struct gfn_to_hva_cache shinfo_cache;
+	gfn_t shinfo_gfn;
 };
 
 enum kvm_irqchip_mode {

arch/x86/kvm/x86.c

@@ -11162,6 +11162,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	kvm_hv_init_vm(kvm);
 	kvm_page_track_init(kvm);
 	kvm_mmu_init_vm(kvm);
+	kvm_xen_init_vm(kvm);
 
 	return static_call(kvm_x86_vm_init)(kvm);
 }

arch/x86/kvm/xen.c

@@ -25,15 +25,14 @@ static int kvm_xen_shared_info_init(struct kvm *kvm, gfn_t gfn)
 {
 	gpa_t gpa = gfn_to_gpa(gfn);
 	int wc_ofs, sec_hi_ofs;
-	int ret;
+	int ret = 0;
 	int idx = srcu_read_lock(&kvm->srcu);
 
-	ret = kvm_gfn_to_hva_cache_init(kvm, &kvm->arch.xen.shinfo_cache,
-					gpa, PAGE_SIZE);
-	if (ret)
+	if (kvm_is_error_hva(gfn_to_hva(kvm, gfn))) {
+		ret = -EFAULT;
 		goto out;
-
-	kvm->arch.xen.shinfo_set = true;
+	}
+	kvm->arch.xen.shinfo_gfn = gfn;
 
 	/* Paranoia checks on the 32-bit struct layout */
 	BUILD_BUG_ON(offsetof(struct compat_shared_info, wc) != 0x900);
@@ -245,7 +244,7 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
 
 	case KVM_XEN_ATTR_TYPE_SHARED_INFO:
 		if (data->u.shared_info.gfn == GPA_INVALID) {
-			kvm->arch.xen.shinfo_set = false;
+			kvm->arch.xen.shinfo_gfn = GPA_INVALID;
 			r = 0;
 			break;
 		}
@@ -283,10 +282,7 @@ int kvm_xen_hvm_get_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
 		break;
 
 	case KVM_XEN_ATTR_TYPE_SHARED_INFO:
-		if (kvm->arch.xen.shinfo_set)
-			data->u.shared_info.gfn = gpa_to_gfn(kvm->arch.xen.shinfo_cache.gpa);
-		else
-			data->u.shared_info.gfn = GPA_INVALID;
+		data->u.shared_info.gfn = gpa_to_gfn(kvm->arch.xen.shinfo_gfn);
 		r = 0;
 		break;
 
@@ -646,6 +642,11 @@ int kvm_xen_hvm_config(struct kvm *kvm, struct kvm_xen_hvm_config *xhc)
 	return 0;
 }
 
+void kvm_xen_init_vm(struct kvm *kvm)
+{
+	kvm->arch.xen.shinfo_gfn = GPA_INVALID;
+}
+
 void kvm_xen_destroy_vm(struct kvm *kvm)
 {
 	if (kvm->arch.xen_hvm_config.msr)

arch/x86/kvm/xen.h

@@ -21,6 +21,7 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data);
 int kvm_xen_hvm_get_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data);
 int kvm_xen_write_hypercall_page(struct kvm_vcpu *vcpu, u64 data);
 int kvm_xen_hvm_config(struct kvm *kvm, struct kvm_xen_hvm_config *xhc);
+void kvm_xen_init_vm(struct kvm *kvm);
 void kvm_xen_destroy_vm(struct kvm *kvm);
 
 static inline bool kvm_xen_msr_enabled(struct kvm *kvm)
@@ -50,6 +51,10 @@ static inline int kvm_xen_write_hypercall_page(struct kvm_vcpu *vcpu, u64 data)
 	return 1;
 }
 
+static inline void kvm_xen_init_vm(struct kvm *kvm)
+{
+}
+
 static inline void kvm_xen_destroy_vm(struct kvm *kvm)
 {
 }