Re: [PATCH v2 20/35] xen/console: introduce console_owner_domid()

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 7 Jan 2025, Jan Beulich wrote:
> On 06.01.2025 19:48, Stefano Stabellini wrote:
> > On Mon, 6 Jan 2025, Jan Beulich wrote:
> >> On 04.01.2025 05:15, Denis Mukhin wrote:
> >>>
> >>> On Tuesday, December 10th, 2024 at 11:28 PM, Jan Beulich 
> >>> <jbeulich@xxxxxxxx> wrote:
> >>>
> >>>>
> >>>>
> >>>> On 06.12.2024 05:41, Denis Mukhin via B4 Relay wrote:
> >>>>
> >>>>> From: Denis Mukhin dmukhin@xxxxxxxx
> >>>>>
> >>>>> console_owner_domid() is introduced to obtain the "console owner" 
> >>>>> domain ID.
> >>>>>
> >>>>> The call is used in NS8250 emulator to identify the case when physical 
> >>>>> xen
> >>>>> console focus is owned by the domain w/ NS8250 emulator, in which case,
> >>>>> messages from guest OS are formatted w/o '(XEN)' prefix.
> >>>>
> >>>>
> >>>> Such messages ought to be processed through guest_printk(), which wants a
> >>>> domain pointer, not a domid_t anyway. Plus isn't that going to be
> >>>> current->domain anyway at the callsite, eliminating the need for such a
> >>>>
> >>>> helper altogether?
> >>>
> >>> If the current domain is owning the physical console and printing, say, 
> >>> Linux
> >>> login prompt, there's no need to add "(XEN)" for every printout; adding 
> >>> timestamps
> >>> can be disabled from Xen command line.
> >>
> >> Surely there shouldn't be (XEN), but without (d<N>) it'll be ambiguous in 
> >> a log
> >> which domain a message came from. As long as only Dom0 messages are left 
> >> un-
> >> prefixed, that's likely fine. Yet as soon as multiple domains can issue 
> >> such
> >> messages (and have console "focus") I think the prefix needs to be there.
> > 
> > It looks like we are aligned on the desired behavior,
> 
> Hmm, no, I don't think we are. I don't ...
> 
> > but for clarity,
> > see https://marc.info/?l=xen-devel&m=173405161613716, also copy/pasted
> > here:
> > 
> > I think we should provide a consistent behavior across architectures.
> > The current behavior with vpl011 and dom0less on ARM is the following:
> > 
> > - no prefix for Dom0 output
> > - DOM$NUM for DomUs when not in focus, otherwise no prefix
> 
> ... view this model as a desirable one. It leaves room for ambiguity.

Adding a few more people in CC for feedback.

My priority is to keep the architectures aligned. It might be OK to
change output format, but then let's do it uniformly on ARM as well.

Jan, please clarify what you think would be better than the above. Is it
the following? I don't think I understood your preference.

- DOM$NUM for Dom0 and DomUs when not in focus, otherwise no prefix