[libvirt] [PATCH 4/4] conf: permit auto-assignment of controller indexes

[Laine Stump]

On 05/15/2016 02:38 PM, Cole Robinson wrote:
> On 05/15/2016 02:30 PM, Laine Stump wrote:
>> On 05/14/2016 10:56 AM, Cole Robinson wrote:
>>> I agree with the goal of the patch, but I think all the index assignment code
>>> should be moved to somewhere in the PostParse call path. The fact that the
>>> controller ParseXML now needs to act on the entire domain def is a giveaway
>>> that it's in the wrong place.
>> I originally did it that way, but there was some problem with it, either
>> actual or imagined/potential. I *think* possibly the problem was that
>> auto-added controllers (which is done in the driver-specific postparse, called
>> prior to the common postparse) are added when an existing controller of the
>> desired index can't be found, but if an index was added in postparse, I would
>> want it to be done in the common postparse (since it is a requirement for
>> *all* drivers);
> Hmm. Most implicit controllers are added in common code actually, however
> there are some added in qemu code it seems, for PCI, which is probably what
> you were referring to.

There are several controllers/devices added which are not just 
hypervisor specific, but specific to particular machinetypes/arches 
within that hypervisor. In particular, qemuDomainDefAddDefaultDevices.

>   In that case we may need to do two passes of index
> assignment, or when adding a PCI controller outside common code we just
> informally require the hv driver to assign the index themselves.

It's not that the new controller needs an index assigned. It's that the 
controller is "maybe added" depending on whether of not there is already 
a controller of the requested type at a particular index (usually 0). 
For example, when we add a default USB controller in 
qemuDomainDefAddDefaultDevices().

As for doing two passes, where would the first pass be run? I don't want 
it to be done in the driver-specific postparse because then it would 
need to be called separately for every driver, which is prone to people 
not doing it. And the common postparse is too late to do any good. The 
only place we're left with, other than in the controller parser itself, 
is  the top-level domain parser function prior to calling the 
driver-specific postparse.

Also there is the case where only a single controller is parsed - the 
toplevel domain parse is never called there in the first place (of 
course I'm skeptical that is ever called for any controller - are any 
controllers hotpluggable?)


> Unfortunately these types of ordering problems are basically unavoidable in
> certain cases... there's no one size fits all ordering for validation, setting
> defaults, adding default devices, and assigning addresses.

Yeah, there's no way to deal with this in the current postparse callback 
framework, and the only way to solve all possible ordering problems in 
that way would be, it seems, to call all of the callbacks repeatedly in 
a loop until it went through an entire cycle without any change :-P

>
> > also is the potential problem that PCI controller indexes must
>> be in place in order for the PCI address auto-assignment to work, and you had
>> previously expressed a desire to move that up into one of the postparse
>> functions rather than having it be a separate function that must be
>> independently called - if that happened, it would also be done in the
>> driver-specific postparse.
> I posted patches yesterday adding address assignment to PostParse, but it
> comes at the very end of the common PostParse usage, to match how
> qemuDomainAssignAddresses is presently used.

Interesting, I need to look at that. Considering that the way addresses 
are assigned could change based on the machinetype/arch, how can you do 
that in the common postparse (which doesn't know about things like qemu 
capabilities)?

>   So at least that bit shouldn't
> interfere with index assignment in generic code