qemu-system-x86_64 appears to have sgx-epc, but not memory-backend-epc

[Markus Armbruster]

My build of qemu-system-x86_64 has device sgx-epc:

 --> {"execute": "qom-list-types", "arguments": {"implements": "sgx-epc"}}
 <-- {"return": [{"name": "sgx-epc", "parent": "device"}]}

The device as a link property to a memory-backend-epc:

 --> {"execute": "qom-list-properties", "arguments": {"typename": "sgx-epc"}}
 <-- {"return": [...
                 {"name": "memdev", "type": "link<memory-backend-epc>"},
                 ...]}

However, type memory-backend-epc does not exist:

 --> {"execute": "qom-list-types", "arguments": {"implements": 
"memory-backend"}}
 <-- {"return": [{"name": "memory-backend-ram", "parent": "memory-backend"}, 
{"name": "memory-backend-file", "parent": "memory-backend"}, {"name": 
"memory-backend-memfd", "parent": "memory-backend"}, {"name": 
"memory-backend-shm", "parent": "memory-backend"}]}

The example from docs/system/i386/sgx.rst

    Launching a guest
    -----------------

    To launch a SGX guest:

    .. parsed-literal::

      |qemu_system_x86| \\
       -cpu host,+sgx-provisionkey \\
       -object memory-backend-epc,id=mem1,size=64M,prealloc=on \\
       -M sgx-epc.0.memdev=mem1,sgx-epc.0.node=0

does not work:

    qemu-system-x86_64: invalid object type: memory-backend-epc

CONFIG_SGX is enabled in my build:

    $ find bld-x86 -name \*.h -exec grep CONFIG_SGX {} +
    bld-x86/x86_64-softmmu-config-devices.h:#define CONFIG_SGX 1
    bld-x86/config-poison.h:#pragma GCC poison CONFIG_SGX