Re: [libvirt] [PATCH 00/13] PCIe fixes + new PCI controllers w/RFC

The first 4 patches are bugfixes/reorganizations that have no controversy. The sets of 5-7, 8-10, and 11-13 each implement a new model of PCI controller: 5-7 - <controller type='pci' model='pcie-root-port'/> This is based on qemu's ioh3420. 8-10 - <controller type='pci' model='pcie-switch-upstream-port'/> Based on qemu's x3130-upstream 11-13 - <controller type='pci' model='pcie-switch-downstream-port'/> (xio3130-downstream) The first patch of each set adds a capability bit for qemu (again non-controversial), the 2nd adds the new pci controller model, and the 3rd implements that model in qemu (by checking for the capability and adding a commandline arg or failing). The "controversial"/RFC bit is this - talking to Alex Williamson (after I'd rwritten these patches, which is why I'm presenting them in a form that I *don't* want to push) about the possibility of qemu adding generic root-port, switch-upstream-port, and switch-downstream-port controllers, and possibly also a generic dmi-to-pci-bridge (i.e. controllers not tied to any particular hardware implementation as with those currently available), I'm realizing that, while it was a correct decision to make all of the existing pci controllers "type='pci'" (since they share an address space), using the "model" attribute to set the kind of controller was probably a mistake. The problem - if: <controller type='pci' model='dmi-to-pci-bridge'/> currently means to add an i82801b11-bridge controller to the domain, once qemu implements a generic dmi-to-pci-bridge, how will *that* be denoted, and how will we avoid replacing the existing i81801b11-bridge in a particular domain with the generic version when a guest is restarted following a qemu/libvirt upgrade? In hindsight, it probably would have been better to do something like this with the four existing pci controllers: <controller type='pci' subType='dmi-to-pci-bridge' model='i82801b11-bridge'/> <controller type='pci' subType='pci-bridge' model='pci-bridge'/> (or maybe blank?) <controller type='pci' subType='pci-root'/> (again maybe model is blank) <controller type='pci' subType='pcie-root'/>(and again) (instead, what is shown above as "subType" is currently placed in the "model" attribute).

Then we could add the 3 new types like this: <controller type='pci' subType='pcie-root-port' model='ioh3420'/> <controller type='pci' subType='pcie-switch-upstream-port' model='x3130-upstream/> <controller type='pci' subType='pcie-switch-downstream-port' model='xio3130-downstream/> and we would easily be able to add support for new generic controllers that behaved identically, by just adding a new model. But we haven't done that, and anything we do in the future must be backwards compatible with what's already there (right?). I'm trying to think of how to satisfy backward compatibility while making things work better in the future. Some ideas, in no particular order: === Idea 1: multiplex the meaning of the "model" attribute, so we currently have: <controller type='pci' model='dmi-to-pci-bridge'/> which means "add an i82801b11-bridge device" and when we add a generic version of this type of controller, we would do it with something like: <controller type='pci' model='generic-dmi-to-pci-bridge'/> and for another vendor's mythical controller: <controller type='pci' model='xyz-dmi-to-pci-bridge'/>

Cons: This will make for ugliness in switch statements where a new case will have to be added whenever a different controller with similar behavior/usage is supported. And it's generally not a good idea to have a single attribute be used for two different functions. === Idea 2: implement new controllers as suggested in "20/20 hindsight" above. For controllers in existing domains (dmi-to-pci-bridge, pic-bridge, pci-root, and pcie-root) imply it into the controller definition of an existing domain when only model has been given (but don't write it out that way, to preserve the ability to downgrade). So this: [1] <controller type='pci' model='dmi-to-pci-bridge'/> would internally mean this: [2] <controller type='pci' subType='dmi-to-pci-bridge' model='i82801b11-bridge'/>

(but would remain as [1] when config is rewritten/migrated) while this: [3] <controller type='pci' subType='dmi-to-pci-bridge' model='anything whatsoever/> would mean exactly what it says. Cons: Keeping this straight would mean having some sort of "oldStyleCompat" flag in the controller object, to be sure that [1] wasn't sent in migration status as [2] (since the destination might not recognize it). It would also mean keeping the code in the parser and formatter to deal with this flag. Forever.

=== Idea 3: interpret controllers with missing subType as above, but actually write it out to the config/migration/etc in the new modified format. Cons: This would prevent downgrading libvirt or migrating from a host with newer libvirt to one with older libvirt. (Although preserving compatibility at some level when downgrading may be a stated requirement of some downstream distros' builds of libvirt, I think for upstream it is only a "best effort"; I'm just not certain how much "best" is considered to be :-)

=== Idea 4: Unlike other uses of "model" in libvirt, for pci controllers, continue to use "model" to denote the subtype/class/whatever of controller, and create a new attribute to denote the different specific implementations of each one. So for example: [4] <controller type='pci' model='dmi-to-pci-bridge'/> would become: [5] <controller type='pci' model='dmi-to-pci-bridge' implementation='i82801b11-bridge'/> (or some other name in place of "implementation" - ideas? I'm horrible at thinkin up names)

Pros: wouldn't create compatibility problems when downgrading or migrating cross version.

Cons: Is inconsistent with every other use of "model" attribute in libvirt, and each new addition of a PCI controller further propagates this misuse.

==== I currently like either option 2 or 3 (depending on how good we want to be about supporting downgrade/intra-version migration), but as is obvious by the fact that it was me that suggested putting the type of pci controller into "model", I am very good at making the wrong decision on matters like this.