Re: [PATCH v4 4/5] vfio/pci: Free resources when vfio_migration_realize fails

["Cédric Le Goater" <clg@redhat.com>]

On 6/29/23 13:45, Joao Martins wrote:
> On 29/06/2023 09:40, Zhenzhong Duan wrote:
>> When vfio_realize() succeeds, hot unplug will call vfio_exitfn()
>> to free resources allocated in vfio_realize(); when vfio_realize()
>> fails, vfio_exitfn() is never called and we need to free resources
>> in vfio_realize().
>>
>> In the case that vfio_migration_realize() fails,
>> e.g: with -only-migratable & enable-migration=off, we see below:
>>
>> (qemu) device_add vfio-pci,host=81:11.1,id=vfio1,bus=root1,enable-migration=off
>> 0000:81:11.1: Migration disabled
>> Error: disallowing migration blocker (--only-migratable) for: 0000:81:11.1: Migration is disabled for VFIO device
>>
>> If we hotplug again we should see same log as above, but we see:
>> (qemu) device_add vfio-pci,host=81:11.1,id=vfio1,bus=root1,enable-migration=off
>> Error: vfio 0000:81:11.1: device is already attached
>>
>> That's because some references to VFIO device isn't released,
>> we should check return value of vfio_migration_realize() and
>> release the references, then VFIO device will be truely
>> released when hotplug fails.
>>
>> Fixes: a22651053b59 ("vfio: Make vfio-pci device migration capable")
>> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
>> ---
>>   hw/vfio/pci.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
>> index 54a8179d1c64..dc69d3031b24 100644
>> --- a/hw/vfio/pci.c
>> +++ b/hw/vfio/pci.c
>> @@ -3210,6 +3210,7 @@ static void vfio_realize(PCIDevice *pdev, Error **errp)
>>           ret = vfio_migration_realize(vbasedev, errp);
>>           if (ret) {
>>               error_report("%s: Migration disabled", vbasedev->name);
>> +            goto out_vfio_migration;
>>           }
>>       }
>>   
>> @@ -3219,6 +3220,8 @@ static void vfio_realize(PCIDevice *pdev, Error **errp)
>>   
>>       return;
>>   
>> +out_vfio_migration:
>> +    vfio_migration_exit(vbasedev);
>>   out_deregister:
>>       vfio_disable_interrupts(vdev);
>>   out_intx_disable:
> 
> I agree with the general sentiment behind the change.
> Clearly vfio::migration and vfio::migration_blocker are leaking from inside the
> migration_realize() function.
> 
> But it is kinda awkward semantic that vfio_migration_realize() (or any realize)
> failures need to be accompanied with a vfio_migration_exit() that tears down
> state *leaked* by its realize() failure.
> 
> It sounds to me that this should be inside the vfio_migration_realize() not on
> the caller? Unless QEMU ::realize() is expected to do this.
> 

I agree. vfio_migration_realize() should handle the cleanup of the resources
it allocated if there is a failure.

Thanks

C.