Re: unable to execute QEMU command 'qom-get': Property 'sgx-epc.unavailable-features' not found

[Yang Zhong <yang.zhong@intel.com>]

On Thu, Nov 25, 2021 at 08:47:22PM +0800, Yang Zhong wrote:
> Hello Paolo,
> 
> Our customer used the Libvirt XML to start a SGX VM, but failed.
> 
> libvirt.libvirtError: internal error: unable to execute QEMU command 'qom-get': Property 'sgx-epc.unavailable-features' not found
> 
> The XML file,
> <qemu:commandline>
>     <qemu:arg value="-cpu"/>
>     <qemu:arg value="host,+sgx,+sgx-debug,+sgx-exinfo,+sgx-kss,+sgx-mode64,+sgx-provisionkey,+sgx-tokenkey,+sgx1,+sgx2,+sgxlc"/>
>     <qemu:arg value="-object"/>
>     <qemu:arg value="memory-backend-epc,id=mem1,size=16M,prealloc=on"/>
>     <qemu:arg value="-M"/>
>     <qemu:arg value="sgx-epc.0.memdev=mem1"/>
>   </qemu:commandline>
> 
> The new compound property command should be located in /machine path,
> which are different with old command '-sgx-epc id=epc1,memdev=mem1'.
> 
> I also tried this from Qemu monitor tool, 
> (qemu) qom-list /machine
> type (string)
> kernel (string)
> ......
> sgx-epc (SgxEPC)
> ......
> sgx-epc[0] (child<memory-region>)
> ......
> 
> We can find sgx-epc from /machine list.
> 

  This issue is clear now, which is caused by Libvirt to get the CPU's unavailable-features by below command:
  {"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"unavailable-features"}

  but in SGX vm, since the sgx is initialized before VCPU because sgx need set the virtual EPC info in the cpuid.  

  So the /machine/unattached/device[0] is occupied by sgx, which fail to get the unvailable-features from
  /machine/unattached/device[0].


  We need fix this issue, but this can be done in Qemu or Libvirt side.

  1) Libvirt side
     If the libvirt support SGX EPCs, libvirt can use /machine/unattached/device[n] to check "unavailable-features".
     n is the next number of sgx's unattached_count.

  2) Qemu side
    
     One temp patch to create one /sgx in the /machine in the device_set_realized() 
diff --git a/hw/core/qdev.c b/hw/core/qdev.c
index 84f3019440..4154eef0d8 100644
--- a/hw/core/qdev.c
+++ b/hw/core/qdev.c
@@ -497,7 +497,7 @@ static void device_set_realized(Object *obj, bool value, Error **errp)
     NamedClockList *ncl;
     Error *local_err = NULL;
     bool unattached_parent = false;
-    static int unattached_count;
+    static int unattached_count, sgx_count;

     if (dev->hotplugged && !dc->hotpluggable) {
         error_setg(errp, QERR_DEVICE_NO_HOTPLUG, object_get_typename(obj));
@@ -509,7 +509,15 @@ static void device_set_realized(Object *obj, bool value, Error **errp)
             goto fail;
         }

-        if (!obj->parent) {
+        if (!obj->parent && !strcmp(object_get_typename(obj), "sgx-epc")) {
+            gchar *name = g_strdup_printf("device[%d]", sgx_count++);
+
+            object_property_add_child(container_get(qdev_get_machine(),
+                                                    "/sgx"),
+                                      name, obj);
+            unattached_parent = true;
+            g_free(name);
+        } else if (!obj->parent) {
             gchar *name = g_strdup_printf("device[%d]", unattached_count++);

             object_property_add_child(container_get(qdev_get_machine()
   
    This patch can make sure vcpu is still /machine/unattached/device[0].


    Which solution is best?  thanks!

    Yang




> I am not familiar with Libvirt side, would you please suggest how to implement
> this compound command in the XML file?  thanks a lot!
> 
> Regards,
> 
> Yang  
>