[libvirt] domXML modeling question

[Jim Fehlig]

There have been a few requests [1][2] to support Xen's max_grant_frames setting 
in libvirt domXML, but I'm not quite sure how to model it. The documentation [3] 
on this setting states:

Specify the maximum number of grant frames the domain is allowed to have.  This
value controls how many pages the domain is able to grant access to for other
domains, needed e.g. for the operation of paravirtualized devices.  The default
is settable via xl.conf(5).

It smells of a <memtune> setting, e.g. the amount of memory a domain can share, 
but doesn't map to any of the existing settings. A new subelement <shared_limit> 
doesn't feel right. Does anyone suggest a better way of modeling max_grant_frames?

Another option I considered is setting the value based on number of PV devices, 
but I think that flies in the face of libvirt's policy of not dictating policy. 
Regardless of domain config modeling I can work on a driver-wide setting in 
libxl.conf, similar to Xen's xl.conf(5) global.

Regards,
Jim

[1] https://www.redhat.com/archives/libvir-list/2018-April/msg00216.html
[2] https://www.redhat.com/archives/libvirt-users/2019-January/msg00011.html
[3] 
http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/man/xl.cfg.5.pod.in;h=ad81af1ed8cc983c76b5ec2c3aa02e28f042cc63;hb=HEAD#l569