Re: [PATCH 4/7] backends/hostmem-epc: Register QOM type unconditionally

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Nov 11, 2026 at 01:23:39PM +0100, Markus Armbruster wrote:
> The example from docs/system/i386/sgx.rst section "Launching a guest"
> fails for me like this:
> 
>     $ qemu-system-x86_64 -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
>     qemu-system-x86_64: invalid object type: memory-backend-epc
> 
> Device "sgx-epc" has a link property to a "memory-backend-epc" (SgxEPC
> member memdev).  It's not optional, so we must create a
> "memory-backend-epc" to be able to create an "sgx-epc".  In the test
> shown above, "sgx-epc" exists, but "memory-backend-epc" does not.
> 
> The latter doesn't exist for me, because we register QOM type
> "memory-backend-epc" only when we can open /dev/sgx_vepc read/write,
> which fails for me.  Such conditional registration is unusual and
> problematic:
> 
> 1. Management applications use introspection interfaces to find out
> what a certain QEMU can do.  Conditional type registration makes QOM
> introspection results depend on the condition, which generally makes
> them valid only for the process that produced them.  Undesirable.
> 
> 2. For user-creatable objects like "memory-backend-epc", it also makes
> QAPI introspection unreliable: query-qmp-schema claims the type
> exists (object-add argument @qom-type accepts "memory-backend-epc")
> even when it doesn't.
> 
> Conditional registration actually serves no purpose here, because we
> still have to open /dev/sgx_vepc for real when we create the object,
> and need and do handle errors then.
> 
> Register unconditionally.  The example now fails to create the object,
> like this:
> 
>     qemu-system-x86_64: can't create 'memory-backend-epc': Could not open '/dev/sgx_vepc': No such file or directory

Bonus points - this error message is orders of magnitude
more useful to users than

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

which makes them think their QEMU is built wrong or outdated.

> 
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>  backends/hostmem-epc.c | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

> 
> diff --git a/backends/hostmem-epc.c b/backends/hostmem-epc.c
> index e1c784249ff..bf5aecdfd45 100644
> --- a/backends/hostmem-epc.c
> +++ b/backends/hostmem-epc.c
> @@ -62,12 +62,7 @@ static const TypeInfo sgx_epc_backed_info = {
>  
>  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);
> -    }
> +    type_register_static(&sgx_epc_backed_info);
>  }
>  
>  type_init(register_types);
> -- 
> 2.55.0
> 

With regards,
Daniel
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