Re: [PATCH v16 1/4] xen/domain: unify domain ID allocation

[dmukhin@xen.org]

On Wed, Aug 20, 2025 at 10:33:16PM +0100, Julien Grall wrote:
> Hi Denis,
> 
> On 12/08/2025 23:30, dmkhn@proton.me wrote:
> > From: Denis Mukhin <dmukhin@ford.com>
> >
> > Currently, there are two different domain ID allocation implementations:
> >
> >    1) Sequential IDs allocation in dom0less Arm code based on max_init_domid;
> >
> >    2) Sequential IDs allocation in XEN_DOMCTL_createdomain; does not use
> >       max_init_domid (both Arm and x86).
> >
> > The domain ID allocation covers dom0 or late hwdom, predefined domains,
> > post-boot domains, excluding Xen system domains (domid >=
> > DOMID_FIRST_RESERVED).
> >
> > It makes sense to have a common helper code for such task across architectures
> > (Arm and x86) and between dom0less / toolstack domU allocation.
> >
> > Note, fixing dependency on max_init_domid is out of scope of this patch.
> >
> > Wrap the domain ID allocation as an arch-independent function domid_alloc() in
> > new common/domid.c based on the bitmap.
> >
> > Allocation algorithm:
> > - If an explicit domain ID is provided, verify its availability and use it if
> >    ID is not used;
> > - If DOMID_INVALID is provided, search the range [1..DOMID_FIRST_RESERVED-1],
> >    starting from the last used ID.
> >    Implementation guarantees that two consecutive calls will never return the
> >    same ID. ID#0 is reserved for the first boot domain (currently, dom0) and
> >    excluded from the allocation range.
> >
> > Remove is_free_domid() helper as it is not needed now.
> >
> > No functional change intended.
> >
> > Signed-off-by: Denis Mukhin <dmukhin@ford.com>
> > Reviewed-by: Julien Grall <jgrall@amazon.com>
>  > Reviewed-by: Alejandro Vallejo <alejandro.garciavallejo@amd.com>
>  > ---> Changes since v15:
> > - fixup for check after the first pass in the bitarray in domid_alloc()
> 
> This was a good catch from Jan. Has a unit-test been added for this issue?

So I looked into how to plumb the test.
The boundary conditions tests are there for IDs: 0, DOMID_FIRST_RESERVED-1,
DOMID_FIRST_RESERVED.

But to actually check that 

        domid_t bound = DOMID_FIRST_RESERVED;

        domid = find_next_zero_bit(domid_bitmap, bound, domid_last + 1);
        if ( domid >= bound ...

I need to write a test harness for bit manipulation and it is a project on its
own to make hypervisor's bitops compiled for the host...

> 
> Anyway, my reviewed-by holds.
> 
> Cheers,
> 
> --
> Julien Grall
>