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

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

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