Re: [PATCH v2 01/17] vfio: Make vfio_devices_dma_logging_start() return bool

[Eric Auger <eric.auger@redhat.com>]

Hi Cédric,

On 6/17/24 14:34, Cédric Le Goater wrote:
> Hello Eric,
>
> On 6/17/24 1:31 PM, Eric Auger wrote:
>> Hi Cédric,
>>
>> On 6/17/24 08:33, Cédric Le Goater wrote:
>>> Since vfio_devices_dma_logging_start() takes an 'Error **' argument,
>>> best practices suggest to return a bool. See the api/error.h Rules
>>> section. It will simplify potential changes coming after.
>>
>>
>> As I already mentionned the Rules section does not say that, as far as I
>> understand:
>> It is allowed to either return a bool, a pointer-value, an integer,
>> along with an error handle:
>>
>> "
>>   *   • bool-valued functions return true on success / false on failure,
>>   *   • pointer-valued functions return non-null / null pointer, and
>>   *   • integer-valued functions return non-negative / negative.
>> "
>>
>> Personally I don't like much returning a bool as I think it rather
>> complexifies the code and to me that kind of change is error prone.
>
> Returning an int could be misleading too, since there are multiple ways
> it could be interpreted. You can find in QEMU routines returning -1 for
> error which is later used as an errno :/
yes no good!
>
> The error framework in QEMU provides a way to to save and return any
> kind of errors, using the error_seg_*() routines. I tend to prefer
> the basic approach: return fail or pass and use the Error parameter
> for details.
>
> Now, the problem, as always, is doing the conversion in all the
> code. This is probably why people, with a kernel background, find
> it confusing.
>
>
>>
>>
>>>
>>> vfio_container_set_dirty_page_tracking() could be modified in the same
>>> way but the errno value can be saved in the migration stream when
>>> called from vfio_listener_log_global_stop().
>>> Signed-off-by: Cédric Le Goater <clg@redhat.com>
>>> Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
>>> ---
>>>   hw/vfio/common.c | 14 +++++++-------
>>>   1 file changed, 7 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
>>> index
>>> 9e4c0cc95ff90209d3e8184035af0806a2bf890b..d48cd9b9361a92d184e423ffc60aabaff40fb487
>>> 100644
>>> --- a/hw/vfio/common.c
>>> +++ b/hw/vfio/common.c
>>> @@ -1020,7 +1020,7 @@ static void
>>> vfio_device_feature_dma_logging_start_destroy(
>>>       g_free(feature);
>>>   }
>>>   -static int vfio_devices_dma_logging_start(VFIOContainerBase
>>> *bcontainer,
>>> +static bool vfio_devices_dma_logging_start(VFIOContainerBase
>>> *bcontainer,
>>>                                             Error **errp)
>>>   {
>>>       struct vfio_device_feature *feature;
>>> @@ -1033,7 +1033,7 @@ static int
>>> vfio_devices_dma_logging_start(VFIOContainerBase *bcontainer,
>>>                                                              &ranges);
>>>       if (!feature) {
>>>           error_setg_errno(errp, errno, "Failed to prepare DMA
>>> logging");
>>> -        return -errno;
>>> +        return false;
>>>       }
>>>         QLIST_FOREACH(vbasedev, &bcontainer->device_list,
>>> container_next) {
>>> @@ -1058,7 +1058,7 @@ out:
>>>         vfio_device_feature_dma_logging_start_destroy(feature);
>>>   -    return ret;
>>> +    return ret == 0;
>>>   }
>>>     static bool vfio_listener_log_global_start(MemoryListener
>>> *listener,
>>> @@ -1067,18 +1067,18 @@ static bool
>>> vfio_listener_log_global_start(MemoryListener *listener,
>>>       ERRP_GUARD();
>>>       VFIOContainerBase *bcontainer = container_of(listener,
>>> VFIOContainerBase,
>>>                                                    listener);
>>> -    int ret;
>>> +    bool ret;
>>>         if (vfio_devices_all_device_dirty_tracking(bcontainer)) {
>>>           ret = vfio_devices_dma_logging_start(bcontainer, errp);
>>>       } else {
>>> -        ret = vfio_container_set_dirty_page_tracking(bcontainer,
>>> true, errp);
>>> +        ret = vfio_container_set_dirty_page_tracking(bcontainer,
>>> true, errp) == 0;
>>
>> why vfio_container_set_dirty_page_tracking(bcontainer, true, errp)
>> doesn't return a bool then?
>
> The errno value can be saved in the migration stream when called from
> vfio_listener_log_global_stop(). So this would require changes in the
> migration subsystem, like we did for vfio_listener_log_global_start().

OK

so although I am not a big fan of that kind of change and once
highlighted this is not mandated by the error.h doc, please feel free to
add my
Reviewed-by: Eric Auger <eric.auger@redhat.com>

Thanks

Eric
>
> Thanks,
>
> C.
>
>
>
>
>
>>
>> Eric
>>
>>>       }
>>>   -    if (ret) {
>>> +    if (!ret) {
>>>           error_prepend(errp, "vfio: Could not start dirty page
>>> tracking - ");
>>>       }
>>> -    return !ret;
>>> +    return ret;
>>>   }
>>>     static void vfio_listener_log_global_stop(MemoryListener *listener)
>>
>