Re: [PATCH] common: don't require use of DOMID_SELF

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 14/01/2021 15:30, Jan Beulich wrote:
> On 14.01.2021 15:43, Julien Grall wrote:
> > On 14/01/2021 14:02, Jan Beulich wrote:
> > > It's not overly difficult for a domain to figure out its ID, so
> > > requiring the use of DOMID_SELF in a very limited set of places isn't
> > > really helpful towards keeping the ID opaque to the guest.
> > So I agree that a domid can be figured out really easily today and in
> > principle it would be fine to relax it.
> >
> > However, most of the guest OSes will care about running on older Xen
> > versions. Therefore they are not going to be able to use this relaxation.
> >
> > So I am not entirely convinced the relaxation is actually worth it for
> > existing hypercalls.
> I'm aware of the concern (Andrew has voiced it both here and in
> earlier contexts), but personally I think undue restrictions should
> not be retained just because they used to be enforced.
I don't disagree about the undue restrictions. My main concern is it 
makes more difficult for a developper to write portable code. The more 
when there is no easy way to find out the differences between Xen 
versions (see more below).
> We've gone
> this same route in a few other occasions not overly long ago, and
> iirc there are two patches going in a similar direction (different
> area of course) that I have still pending and which neither got
> ack-ed nor firmly rejected. >> Anyway, if we decide to relax it, then I think 
> we should update the
> > public headers because an OS using this relaxation will not work on
> > older Xen. A developper will not be able to know that without looking at
> > the implementation.
> Well, DOMID_SELF exists because that's the preferred form to use.
> I can certainly add commentary, but it would feel a little odd to
> do so.
Lets imagine your are the developer of a new OS but don't know Xen 
internal. How would you find out the difference between Xen interfaces?
With no documentation you have two choices:
   1) Dig into Xen code to understand the parameters
   2) Rely on the testing to find interface

Neither of the two solutions are great for the developper.

> To be honest I'm also not sure how helpful this is going to
> be, considering that consumers often have their own clones of our
> headers.
Right, but IMHO, anyone writing code that interface with the hypervisor 
should read the latest documentation/interface.
At the moment, most of our documentations are in the public headers. So 
it makes a suitable place.
We may need to duplicate the comment, but it is small enough to be fine.

Cheers,

--
Julien Grall