Re: [PATCH libvirt v1 0/6] Fix ZPCI address auto-generation on s390

On Wed, 2020-05-13 at 17:32 +0100, Daniel P. Berrangé wrote: > On Tue, May 12, 2020 at 12:13:22PM +0200, Boris Fiuczynski wrote: > > The behavior change would be > > Current code: > > uid=0 fid=0 -> uid=0 fid=0 -> address gets autogenerated > > uid=0 fid=x -> uid=0 fid=x -> address is rejected as invalid > > uid=0 -> uid=0 fid=0 -> address gets autogenerated > > IIUC, in the two cases here where the address gets auto-generated, > the resulting guest VM successfully boots & runs.... > > > With the series applied > > uid=0 fid=0 -> uid=0 fid=0 -> address is rejected as invalid > > uid=0 fid=x -> uid=0 fid=x -> address is rejected as invalid > > uid=0 -> uid=0 fid=0 -> address is rejected as invalid > > ...so this proposed change is a functional regression for the > user. > > > The documentation already specifies the uid value range correctly. > > The fix for users hitting the two scenarios (uid=0 fid=0) and (uid=0) is > > simple: Remove the zpci definition completely. > > This would be taking a users' currently working VM, intentionally > breaking it, and then making the user pick up the pieces. This is > an example of a behaviour regression that libvirt promises to not > do to users. The bit of nuance that might be missing here is that existing guests already have a full zPCI address stored in the domain XML, which means the wouldn't be affected in any way; additionally, the case where no zPCI address is provided when defining a new guest, which I assume is the most common one, will keep working. The only scenarios that would no longer work are: * the user manually specifies uid=0 fid=0; * the user manually specifies uid=0 and doesn't specify fid. In both cases the user would have gone out of their way to specify a value for the uid attribute that is documented as being invalid: PCI addresses for S390 guests will have a zpci child element, with two attributes: uid (a hex value between 0x0001 and 0xffff [...] https://libvirt.org/formatdomain.html#elementsAddress

As a result, they'd now get a pretty clear error message at define time instead of confusing behavior across the board. I'm not really sure anyone would complain about such a change.