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

[Markus Armbruster <armbru@redhat.com>]

Peter Maydell <peter.maydell@linaro.org> writes:

> On Fri, 23 Oct 2026 at 11:21, Daniel P. Berrangé <berrange@redhat.com> wrote:
>>
>> On Thu, Oct 22, 2026 at 02:58:16PM +0200, Markus Armbruster wrote:
>> > 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:
>
>> There is some rather dubious code that conditionally registers
>> the type
>>
>> static void register_types(void)
>> {
>>     int fd = qemu_open_old("/dev/sgx_vepc", O_RDWR);
>>     if (fd >= 0) {
>>         close(fd);
>>
>>         type_register_static(&sgx_epc_backed_info);
>>     }
>> }
>>
>>
>> Normal practice is for types to be unconditionally registered.  Do we
>> have any examples outside of this SGX case where we conditionally
>> register types ?   Should we change this to follow normal practice ?
>
> TYPE_MEMORY_BACKEND_MEMFD does the same. I did a quick grep for
> "calls to type_register not indented by exactly 4 spaces" and
> I think the other cases are "misindented" or "data-driven
> type registration" rather than trying to do things conditionally
> (but I didn't actually look at all of them, instead making
> guesses based on variable names, so maybe there's one or two
> more I missed).

Alright, I looked.  In addition to "misindented" or "data-driven", I
found:

* register_types() in hostmem-memfd.c registers "memory-backend-memfd"
  only when a test of memfd_create() succeeds.  This is what Peter
  reported.

* virtio_ccw_gpu_register() in hw/s390x/virtio-ccw-gpu.c registers
  "virtio-gpu-ccw" only when have_virtio_ccw, which is true when
  hw/s390x/virtio-ccw.c gets compiled into the binary.  So it's
  effectively compile-time conditional.

  I wish it was literally compile-time conditional, either in meson or
  using #if.

* qemu_console_early_init() in ui/console-vc.c registers "chardev-vc" if
  it's not already registered.  early_dbus_init() in ui/dbus.c,
  early_gtk_display_init() in ui/gtk.c, and
  spice_app_display_early_init() in ui/spice-app.c all register it.

  These are all QemuDisplay .early_init() callbacks.
  qemu_display_early_init() calls the selected display's .early_init(),
  if non-null.

  Because qemu_console_early_init() runs after
  qemu_display_early_init(), this always registers a "chardev-vc": the
  active display may register its own, and when it doesn't, we get
  ui/console-vc.c's.

  More moving parts than I'd like.

I'm going to post patches for backends/hostmem-{epc,memfd}.c.  I don't
plan to clean up the other two at this time.

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