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

[Jan Beulich <jbeulich@suse.com>]

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. 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. 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.

Jan