Re: [PATCH v4] xen/domain: introduce DOMID_ANY

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Feb 04, 2026 at 08:56:10AM +0100, Jan Beulich wrote:
> On 04.02.2026 08:49, Roger Pau Monné wrote:
> > On Fri, Jan 09, 2026 at 06:07:48AM -0800, dmukhin@xxxxxxx wrote:
> >> --- a/xen/include/public/xen.h
> >> +++ b/xen/include/public/xen.h
> >> @@ -608,6 +608,11 @@ DEFINE_XEN_GUEST_HANDLE(mmuext_op_t);
> >>  /* DOMID_INVALID is used to identify pages with unknown owner. */
> >>  #define DOMID_INVALID        xen_mk_uint(0x7FF4)
> >>  
> >> +#if defined(__XEN__) || defined(__XEN_TOOLS__)
> >> +/* Domain ID allocator: search [1..DOMID_FIRST_RESERVED-1] range. */
> >> +#define DOMID_ANY            xen_mk_uint(0x7FF5)
> >> +#endif
> > 
> > I would attempt to word the comment in a more generic way.  While this
> > is now only used for the domain ID allocator, it's likely to gain more
> > uses going forward (Juergen already expressed interest):
> > 
> > "DOMID_ANY is used to signal no specific domid requested.  Handler
> > should pick a valid domid, or handle it as a broadcast value depending
> > on the context."
> 
> +1
> 
> > Also, I would remove the tools guards, I think once a DOMID_ constant
> > is allocated it becomes part of the public ABI, and it cannot be
> > withdrawn.  See for example DOMID_IDLE: it's only used internally in
> > the hypervisor AFAICT, yet the define is fully visible in the
> > headers.
> 
> It was me to ask for it to be guarded like this. DOMID_IDLE (and perhaps
> others) not being guarded (at least for IDLE: by just __XEN__) imo was a
> mistake. That mistake may in fact be correctable, if we could prove that
> the ID cannot usefully be passed into anywhere.

Even if it's not passed into anything, does it make sense to guard
them?  The reserved domid values are already consumed, ie: cannot be
reused in any way.  It just seem to me like more ifdefery churn for no
specific benefit.

Thanks, Roger.