[libvirt] ambiguous ret of qemuDomainDetachVirtioDiskDevice

zhang bo oscar.zhangbo at huawei.com
Thu Jul 30 09:41:52 UTC 2015

On 2015/7/28 16:33, Ján Tomko wrote:

> On Tue, Jul 28, 2015 at 04:25:13PM +0800, zhang bo wrote:
>> static int
>> qemuDomainDetachVirtioDiskDevice(virQEMUDriverPtr driver,
>>                                  virDomainObjPtr vm,
>>                                  virDomainDiskDefPtr detach)
>> {
>> .......
>>     rc = qemuDomainWaitForDeviceRemoval(vm);
>>     if (rc == 0 || rc == 1)
>>         ret = qemuDomainRemoveDiskDevice(driver, vm, detach);
>>     else
>>         ret = 0;  /*the return value of 2 is dismissed here, which refers to ETIMEOUT.*/
>> ........
>> }
>> ------------------------------------
>> If it timeouts when qemu tries to del the device, the return value would be modified from 2 to 0 in 
>> function qemuDomainDetachVirtioDiskDevice(), which means that, the users would be misleaded that 
>> the device has been deleted, however, the device maybe probably failed to be detached after timeout and
>> still in use. 
> This is intentional and documented:
> http://libvirt.org/html/libvirt-libvirt-domain.html#virDomainDetachDeviceFlags
> Unplugging a disk requires guest cooperation, so the best we can do is
> ask qemu to unplug it and wait for a while.
>> That is to say, the function qemuDomainDetachVirtioDiskDevice()'s return value is ambiguous when it's 0, 
>> maybe successful, or timeout. Will it be better to pass ETIMEOUT to user? or any other advises? for example,
>> let users themselves dumpxml the guest to check whether the device has been actually detached or not? 
> Either dump the XML, or wait for the VIR_DOMAIN_EVENT_ID_DEVICE_REMOVED
> event, as the API documentation suggests.
> Jan

It seems to have fixed the problem by dumping the XML or wait for the DEVICE_REMOVED event. However, it seems to 
make nova or other apps to do more checking work, they need to dump XML or wait the event even if the device has
already been successfully removed. which is unnecessary. 

I think, it's better to return ETIMEOUT and let nova to dumpxml or wait the event at this situation, rather than always
doing that job.

It maybe a better design, what's your opinion?

oscar.zhangbo at huawei.com  

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