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

[Julien Grall <julien@xxxxxxx>]

Hi Denis,

On 10/06/2025 10:04, dmkhn@xxxxxxxxx wrote:
> On Tue, Jun 10, 2025 at 10:26:22AM +0200, Jan Beulich wrote:
> > On 10.06.2025 10:02, dmkhn@xxxxxxxxx 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@xxxxxxxxx wrote:
> > > > > > > From: Denis Mukhin <dmkhn@xxxxxxxxx>
> > > > > > >
> > > > > > > From: Denis Mukhin <dmukhin@xxxxxxxx>
> > > > > > >
> > > > > > > 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@xxxxxxxx>
> > > > > > > ---
> > > > > > > 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
I am with Jan here. This used to work before without "control_domid", so 
we ought to keep the same.
But even if we are ok to break compatibility, I don't see the value of 
"control_domid". The implication of setting "hardware_domid" is you will
have a separate control domain. At which point, why would it matter to 
specify the domain ID?
Cheers,

--
Julien Grall