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

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.

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