KVM: x86/xen: re-initialize shared_info if guest (32/64-bit) mode is set

If the shared_info PFN cache has already been initialized then the content
of the shared_info page needs to be re-initialized whenever the guest
mode is (re)set.
Setting the guest mode is either done explicitly by the VMM via the
KVM_XEN_ATTR_TYPE_LONG_MODE attribute, or implicitly when the guest writes
the MSR to set up the hypercall page.
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lore.kernel.org/r/20240215152916.1158-12-paul@xen.orgSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/xen.c

@@ -625,8 +625,16 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
 		} else {
 			mutex_lock(&kvm->arch.xen.xen_lock);
 			kvm->arch.xen.long_mode = !!data->u.long_mode;
+
+			/*
+			 * Re-initialize shared_info to put the wallclock in the
+			 * correct place. Whilst it's not necessary to do this
+			 * unless the mode is actually changed, it does no harm
+			 * to make the call anyway.
+			 */
+			r = kvm->arch.xen.shinfo_cache.active ?
+				kvm_xen_shared_info_init(kvm) : 0;
 			mutex_unlock(&kvm->arch.xen.xen_lock);
-			r = 0;
 		}
 		break;
 
@@ -1101,9 +1109,24 @@ int kvm_xen_write_hypercall_page(struct kvm_vcpu *vcpu, u64 data)
 	u32 page_num = data & ~PAGE_MASK;
 	u64 page_addr = data & PAGE_MASK;
 	bool lm = is_long_mode(vcpu);
+	int r = 0;
+
+	mutex_lock(&kvm->arch.xen.xen_lock);
+	if (kvm->arch.xen.long_mode != lm) {
+		kvm->arch.xen.long_mode = lm;
+
+		/*
+		 * Re-initialize shared_info to put the wallclock in the
+		 * correct place.
+		 */
+		if (kvm->arch.xen.shinfo_cache.active &&
+		    kvm_xen_shared_info_init(kvm))
+			r = 1;
+	}
+	mutex_unlock(&kvm->arch.xen.xen_lock);
 
-	/* Latch long_mode for shared_info pages etc. */
-	vcpu->kvm->arch.xen.long_mode = lm;
+	if (r)
+		return r;
 
 	/*
 	 * If Xen hypercall intercept is enabled, fill the hypercall