Re: [PATCH 01/11] xen: reserve flags for internal usage in xen_domctl_createdomain

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 28/09/2021 13:05, Jan Beulich wrote:
> On 23.09.2021 11:54, Julien Grall wrote:
> > On 23/09/2021 08:11, Penny Zheng wrote:
> > > From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
> > >
> > > We are passing an extra special boolean flag at domain creation to
> > > specify whether we want to the domain to be privileged (i.e. dom0) or
> > > not. Another flag will be introduced later in this series.
> > >
> > > Reserve bits 16-31 from the existing flags bitfield in struct
> > > xen_domctl_createdomain for internal Xen usage.
> > I am a bit split with this approach. This feels a bit of a hack to
> > reserve bits for internal purpose in external headers. But at the same
> > time I can see how this is easier to deal with it over repurposing the
> > last argument of domain_create().
> I actually have trouble seeing why that's easier. It is a common thing
> to widen a bool to "unsigned int flags" when more than one control is
> needed.
I was suggesting this is easier for the following two reasons:
  1) All the option flags (internal and external) are in a single place.
  2) Reduce the risk to make a mistake when widening the field. In 
particular in the context of backporting. Although, this looks unlikely 
here.
> Plus this makes things needlessly harder once (in the future)
> the low 16 bits are exhausted in the public interface.
That's why I suggested this sounds like a hack. At the same time the 
split between external vs internal option is a bit more a pain for the 
developper. So I didn't feel pushing for one vs the other. That said, I 
will not argue against if you want to push for repurposing the last 
argument.
Cheers,

--
Julien Grall