KVM: x86: Only advertise KVM_CAP_READONLY_MEM when supported by VM

Until recently, KVM_CAP_READONLY_MEM was unconditionally supported on
x86, but this is no longer the case for SEV-ES and SEV-SNP VMs.

When KVM_CHECK_EXTENSION is invoked on a VM, only advertise
KVM_CAP_READONLY_MEM when it's actually supported.

Fixes: 66155de9 ("KVM: x86: Disallow read-only memslots for SEV-ES and SEV-SNP (and TDX)")
Cc: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Michael Roth <michael.roth@amd.com>
Signed-off-by: Tom Dohrmann <erbse.13@gmx.de>
Message-ID: <20240902144219.3716974-1-erbse.13@gmx.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -4656,7 +4656,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_ASYNC_PF_INT:
 	case KVM_CAP_GET_TSC_KHZ:
 	case KVM_CAP_KVMCLOCK_CTRL:
-	case KVM_CAP_READONLY_MEM:
 	case KVM_CAP_IOAPIC_POLARITY_IGNORED:
 	case KVM_CAP_TSC_DEADLINE_TIMER:
 	case KVM_CAP_DISABLE_QUIRKS:
@@ -4815,6 +4814,8 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_VM_TYPES:
 		r = kvm_caps.supported_vm_types;
 		break;
+	case KVM_CAP_READONLY_MEM:
+		r = kvm ? kvm_arch_has_readonly_mem(kvm) : 1;
 	default:
 		break;
 	}