On Wed, May 13, 2020 at 07:41:34PM +0200, Andrea Bolognani wrote:
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 [...]
The effect of specifying zero though is that we perform allocation to assign a non-zero address, which is then valid. The same happens with regular PCI devices if you give slot="0".
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.
I don't see this existing behaviour as confusing. It looks like mostly being a docs ommission about auto-allocation taking place. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|