Re: [PATCH v9 2/3] xen/domain: adjust domain ID allocation for Arm

[dmkhn@proton.me]

On Tue, Jun 10, 2025 at 10:26:22AM +0200, Jan Beulich wrote:
> On 10.06.2025 10:02, dmkhn@proton.me wrote:
> > On Tue, Jun 10, 2025 at 08:53:12AM +0200, Jan Beulich wrote:
> >> On 06.06.2025 23:29, Julien Grall wrote:
> >>> Hi Denis,
> >>>
> >>> On 05/06/2025 23:05, Julien Grall wrote:
> >>>> Hi Denis,
> >>>>
> >>>> On 28/05/2025 23:50, dmkhn@proton.me wrote:
> >>>>> From: Denis Mukhin <dmkhn@proton.me>
> >>>>>
> >>>>> From: Denis Mukhin <dmukhin@ford.com>
> >>>>>
> >>>>> Remove the hardcoded domain ID 0 allocation for hardware domain and replace it
> >>>>> with a call to get_initial_domain_id() (returns the value of hardware_domid on
> >>>>> Arm).
> >>>>
> >>>> I am not entirely why this is done. Are you intending to pass a different domain ID? If so...
> >>>>
> >>>>>
> >>>>> Update domid_alloc(DOMID_INVALID) case to ensure that get_initial_domain_id()
> >>>>> ID is skipped during domain ID allocation to cover domU case in dom0less
> >>>>> configuration. That also fixes a potential issue with re-using ID#0 for domUs
> >>>>> when get_initial_domain_id() returns non-zero.
> >>>>>
> >>>>> Signed-off-by: Denis Mukhin <dmukhin@ford.com>
> >>>>> ---
> >>>>> Changes since v8:
> >>>>> - rebased
> >>>>> ---
> >>>>>   xen/arch/arm/domain_build.c             | 4 ++--
> >>>>>   xen/common/device-tree/dom0less-build.c | 9 +++------
> >>>>>   xen/common/domain.c                     | 4 ++--
> >>>>>   3 files changed, 7 insertions(+), 10 deletions(-)
> >>>>>
> >>>>> diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
> >>>>> index e9d563c269..0ad80b020a 100644
> >>>>> --- a/xen/arch/arm/domain_build.c
> >>>>> +++ b/xen/arch/arm/domain_build.c
> >>>>> @@ -2035,9 +2035,9 @@ void __init create_dom0(void)
> >>>>
> >>>> ... naming like create_dom0() probably wants to be renamed.
> >>>>
> >>>> That said, I am not convinced a domain other than 0 should have full privilege by default. So I would argue it should stay as ...
> >>>>
> >>>>>       if ( !llc_coloring_enabled )
> >>>>>           flags |= CDF_directmap;
> >>>>> -    domid = domid_alloc(0);
> >>>>> +    domid = domid_alloc(get_initial_domain_id());
> >>>>
> >>>> ... 0.
> >>>
> >>> Looking at the implementation of get_initial_domain_id(), I noticed the behavior was changed for x86 by [1].
> >>>
> >>> Before, get_initial_domain_id() was returning 0 except for the PV shim.
> >>> But now, it would could return the domain ID specified on the command line (via hardware_dom).
> >>>
> >>> From my understanding, the goal of the command line was to create the hardware domain *after* boot. So initially we create dom0 and then initialize the hardware domain. With the patch below, this has changed.
> >>>
> >>> However, from the commit message, I don't understand why. It seems like we broke late hwdom?
> >>>
> >>> For instance, late_hwdom_init() has the following assert:
> >>>
> >>>     dom0 = rcu_lock_domain_by_id(0);
> >>>     ASSERT(dom0 != NULL);
> >>>
> >>> Jan, I saw you were involved in the review of the series. Any idea why this was changed?
> >>
> >> I simply overlooked this aspect when looking at the change. You're right, things
> >> were broken there. Unless a simple and clean fix can be made relatively soon, I
> >> think this simply needs reverting (which may mean to revert any later commits
> >> that depend on that). I can't help noting that in this console rework there were
> >> way too many issues, and I fear more than just this one may have slipped
> >> through. I therefore wonder whether taken as a whole this was/is worth both the
> >> submitter's and all the reviewers' time.
> >
> > Yes, sorry, I overlooked late_hwdom_init() modification.
> >
> > IMO, the clean fix would be adding another command line parameter
> > `control_domid` (with default value 0), make get_initial_domain_id() return it
> > instead of current `hardware_domid` and update late_hwdom_init() to use
> > `control_domid` insted of open-coded 0.
> 
> No, no new command line option will address this. Original behavior needs to be
> restored (either by correcting the earlier change or, as said, be reverting).

Correction of the earlier change:

  https://gitlab.com/xen-project/people/dmukhin/xen/-/commit/b7f194650307a08a9e6da5aa9fdd1f8a9afd67eb

re: command line option: I meant something like this:

  https://gitlab.com/xen-project/people/dmukhin/xen/-/commit/51a519de6ea73ff3b650fd9bd4f3c5c63f64c069

> 
> Jan