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

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
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.


Julien Grall



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