[libvirt] Question about hypervisor <features> that are not tristate

[Jim Fehlig]

Happy new year!

Nearly a year ago I reported an issue with the <hap> hypervisor feature on Xen 
[1] and am now seeing a similar issue with the <pae> feature. Setting the 
default value of pae changed between xend and libxl. When not specified, xend 
would enable pae for HVM domains. Clients such as xm and the old libvirt driver 
did not have to explicitly enable it. In libxl, the pae field within 
libxl_domain_build_info is initialized to 0. Clients must enable pae, and indeed 
xl will do so if pae=1 is not specified in the xl.cfg.

The xend behavior prevents libvirt from disabling pae, whereas the libxl behvior 
causes a guest ABI change (config that worked with libvirt+xend doesn't with 
libvirt+libxl). The libxl behavior also forces management software (e.g. 
OpenStack nova) to add <pae> where it wasn't needed before.

To solve this problem for <hap>, it was changed it to a tristate [2], allowing 
it to be turned off with explicit <hap state='off'/>, and on if not specified or 
<hap/> or <hap state='on'/>. Should <pae> (and the remaining hypervisor features 
that are not tristate) be converted to tristate similar to <hap>? Alternatively, 
I could simply set pae=1 for all HVM domains in the libxl driver. Like the old 
libvirt+xend behavior it couldn't be turned off, but I don't think there is a 
practical use-case to do so. At least no one has complained over all the years 
of libvirt+xend use.

Regards,
Jim

[1] https://www.redhat.com/archives/libvir-list/2016-February/msg00197.html
[2] https://www.redhat.com/archives/libvir-list/2016-March/msg00001.html