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

[Duan, Zhenzhong]

>-----Original Message-----
>From: Joao Martins <joao.m.martins@oracle.com>
>Subject: Re: [PATCH v4 4/5] vfio/pci: Free resources when
>vfio_migration_realize fails
>
>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.
Good suggestion, will fix.

Thanks
Zhenzhong