[libvirt] [PATCH 1/2] conf: nodedev: Don't refresh host caps in testdriver

[John Ferlan]

On 03/02/2018 04:36 PM, Cole Robinson wrote:
> On 03/02/2018 04:02 PM, John Ferlan wrote:
>>
>>
>> On 02/23/2018 06:16 PM, Cole Robinson wrote:
>>> Add a 'testdriver' bool that we set for test_driver.c nodedevs
>>> which will skip accessing host resources via virNodeDeviceUpdateCaps
>>>
>>> Signed-off-by: Cole Robinson <crobinso at redhat.com>
>>> ---
>>>  src/conf/node_device_conf.c | 3 +++
>>>  src/conf/node_device_conf.h | 1 +
>>>  src/test/test_driver.c      | 2 ++
>>>  3 files changed, 6 insertions(+)
>>>
>>> diff --git a/src/conf/node_device_conf.c b/src/conf/node_device_conf.c
>>> index fd8f4e4a9..90c940f11 100644
>>> --- a/src/conf/node_device_conf.c
>>> +++ b/src/conf/node_device_conf.c
>>> @@ -2425,6 +2425,9 @@ virNodeDeviceUpdateCaps(virNodeDeviceDefPtr def)
>>>  {
>>>      virNodeDevCapsDefPtr cap = def->caps;
>>>  
>>> +    if (def->testdriver)
>>> +        return 0;
>>> +
>>>      while (cap) {
>>>          switch (cap->data.type) {
>>>          case VIR_NODE_DEV_CAP_SCSI_HOST:
>>> diff --git a/src/conf/node_device_conf.h b/src/conf/node_device_conf.h
>>> index 685ae3034..665f766e2 100644
>>> --- a/src/conf/node_device_conf.h
>>> +++ b/src/conf/node_device_conf.h
>>> @@ -316,6 +316,7 @@ struct _virNodeDeviceDef {
>>>      char *driver;                       /* optional driver name */
>>>      char *devnode;                      /* /dev path */
>>>      char **devlinks;                    /* /dev links */
>>> +    bool testdriver;                    /* if true, skip host checks */
>>
>> Not sure this should be in virNodeDeviceDef... I think it should be in
>> virNodeDeviceObj. Yes, a bit more work, but I think cleaner. You'd need
>> to create an accessor function in order to set the flag from
>> test_driver. Then avoid calling virNodeDeviceUpdateCaps only from
>> virNodeDeviceMatch if the flag is set.
>>
>> Also instead of "testdriver", how about "skipUpdateCaps" since the
>> purpose of this is to skip calling virNodeDeviceUpdateCaps?
>>
>> At least that hides that this currently is only for the test driver.
>> Perhaps in the future there could be some other reason to not want to
>> update the caps for some specific definition after perhaps it's "known"
>> or "determined" that a specific update had occurred.
> 
> Okay, thanks for review. How about finding a way to remove UpdateCaps
> from generic conf.c implementations instead? Seems wrong that something
> functions needed to implement ListAllDevices touch host resources, I
> don't think other objects work like that
> 

The call to add update for export was a fairly recent, commit id
'd18feadc'. Seems that Erik Skultety has the most recent knowledge.
Hopefully he reads and chimes in...

The nodedev driver isn't like other drivers w/r/t being able to define
or create defs on your own. Instead you rely on udev (or gasp hal) in
order to provide "events" that would add, change, delete the device and
get it's capabilities.

It gets worse because even though an event is fired, it doesn't mean the
update has really occurred. I know mdevs and npiv scsi_host/target's are
afflicted by similar problems - udev says it's got an event, but some
other layer such as systemd is still filling in the data. For npiv
devices, I found that there was a possibility if the timing was just
right that the wwnn, wwpn, and fabric_wwn fields were set to 0 or
ffffffffffffffff, see bz 1319544 for some details. I did generate a
patch to work around it, but it wasn't accepted because it was a hack or
workaround essentially. I know I've reviewed a patch recently in the
mdev space for a somewhat similar problem.

I wonder if it would be better to have some sort of refresh logic that
would do the same/similar type magic at least with respect to getting
updates for the 4 types (SCSI_HOST, SCSI_TARGET, NET, PCI_DEV) that seem
to be lagging or needing to get the latest information. Ironically I
would half expect MDEV or MDEV_TYPES to be included in the need update
list if only because of the similar issues to npiv at least w/r/t
systemd interaction. In any case, I'm not sure there's a simple way to
"know" your data is out of date.

Perhaps the UpdateCaps call for Export isn't necessary, but I can see a
reason for it - that "automagic update" functionality. Just because it
doesn't work well for the test driver isn't perhaps a "good enough"
reason to just remove it.

I'm willing to be convinced otherwise though!

John