Re: [Qemu-devel] [PATCH 7/9] qdev: Use QError for not found error

[Gerd Hoffmann <kraxel@redhat.com>]

> I think just returning error codes to the client is far too little
> information. I don't think we need the fully normalized structure
> that Luiz originally proposed with bus/dev addresses split out, but
> we certainly need to include a string description giving as much
> detail as possible.  If attaching a host USB device failed, I don't
> want a single error code  QERR_OPEN_FAILED, nor a generic message
> like 'could not open device', i want the exact details, eg
>
>    'could not open device: permission denied'
>    'could not open device: no such file or directory'
>    'could not open device: device or resource busy'

Which makes me wonder whenever it makes sense to re-use errno for the 
error codes instead of inventing our own QERR_* codes?  Not the numbers 
of course because they are not standardized as far I know, but the Ename 
strings.

If you error out because a system call failed you can pass up errno 
straight to the client without loosing information along the way.  And 
for other error conditions it should be possible to find error codes 
describing what happened, i.e. when trying to add a device where no 
backend driver exists in qemu you can return ENOENT (plus freeform text 
message for error logging).

cheers,
   Gerd