Re: [Qemu-block] [Qemu-devel] [PATCH for-2.6 0/2] Preparation for PCI devices convert to realize()

[Markus Armbruster]

Cao jin <address@hidden> writes:

> Hi John
>
> On 12/05/2015 01:55 AM, John Snow wrote:
>>
>>
>> On 12/04/2015 02:47 AM, Cao jin wrote:
>>> Hi,
>>>      As you know, there are many PCI devices still using .init() as its
>>>      initialization function, I am planning to do the "convert to realize()"
>>>      work, and PCI bridge devices are chosen first.
>>>      The supporting functions should be modified first. msi_init() a 
>>> supporting
>>>      function for PCI devices.
>>>
>>>      Maybe it should be put in 2.6, as title indicated
>>>
>>> Cao jin (2):
>>>    Add param Error** to msi_init()
>>>    Modify callers of msi_init()
>>>
>>>   hw/audio/intel-hda.c               |  7 ++++++-
>>>   hw/ide/ich.c                       |  2 +-
>>>   hw/net/vmxnet3.c                   |  3 ++-
>>>   hw/pci-bridge/ioh3420.c            |  6 +++++-
>>>   hw/pci-bridge/pci_bridge_dev.c     |  6 +++++-
>>>   hw/pci-bridge/xio3130_downstream.c |  7 ++++++-
>>>   hw/pci-bridge/xio3130_upstream.c   |  7 ++++++-
>>>   hw/pci/msi.c                       | 17 +++++++++++++----
>>>   hw/scsi/megasas.c                  |  2 +-
>>>   hw/scsi/vmw_pvscsi.c               |  3 ++-
>>>   hw/usb/hcd-xhci.c                  |  5 ++++-
>>>   hw/vfio/pci.c                      |  3 ++-
>>>   include/hw/pci/msi.h               |  4 ++--
>>>   13 files changed, 55 insertions(+), 17 deletions(-)
>>>
>>
>> You'll need to squash these patches as the first patch will break git
>> bisect.
>>
>
> Ok, will squash it. And I have another question： what`s the benefit
> of converting to realize? Because AFAICT, doing this make the error
> reporting machanism seems clean & clear, all device-init errors are
> passed above along the call chain. I mean, besides, are there any
> other benefits?

Let's compare behavior on error:

* realize methods pass an error object to their caller

  The caller decides how to handle the error.

  Many callers simply pass it on.  Typically, errors from realize are
  passed up the call chain through qdev_device_add().

  Eventually, the caller that handles the error is reached.  If the
  error needs to be reported to the user, it decides how.  Examples:

  - device_init_func(), which does the actual work for -device, reports
    the human-readable message to stderr.

  - hmp_device_add(), which does the work for HMP device_add, reports
    the human-readable message to to the appropriate monitor.

  - handle_qmp_command(), which handles all QMP commands including QMP
    device_add, sends a QMP error reply down the QMP connection.

  - petalogix_ml605_init(), which creates the "petalogix-ml605" board,
    aborts when realizing the CPU fails.

* init methods report to stderr and return -1

  Fine when the error should be reported to stderr.  This is fine for
  some of the above examples, and wrong on others.  In particular, a
  device needs to provide a realize method to be fully work with
  device_add, unless its init method cannot fail.