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

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

On Wed, Jan 08, 2025 at 09:13:02AM +0100, Jan Beulich wrote:
> On 08.01.2025 09:04, Roger Pau Monné wrote:
> > On Wed, Jan 08, 2025 at 08:28:32AM +0100, Jan Beulich wrote:
> >> On 08.01.2025 00:40, Stefano Stabellini wrote:
> >>> 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
> >>
> >> No, I mean like we have it with guest_printk() today. (XEN) for Xen's
> >> own messages, (d<N>) for ordinary domains' ones, and no prefix
> >> exclusively for the hardware/control domain. What is best to do when
> >> hardware and control domains are distinct I'm uncertain - I'd be
> >> inclined to suggest that the hardware domain then stay the one without
> >> any prefix.
> > 
> > One concern I have with this approach is whether the addition of the
> > (d<N>) prefixes will skew output of interactive applications.  So far
> > the prefix is added to output from all domains different than dom0
> > because the console is not interactive for them, and hence no input
> > can be consumed.
> 
> Hmm, that's an aspect I have to admit I didn't think of.
> 
> > If that changes however, and domains different than dom0 can get input
> > from the Xen console then I wonder how much the added prefix will skew
> > output.  Another possible option would be to not print the prefix for
> > the domain that has the console input assigned (current target), and
> > print it for all other domains (even for dom0 when not in focus).
> 
> That's largely what aiui was proposed. My extra requirement there would
> then be that we make sure a log message is always emitted when console
> focus shifts, so it's possible to identify the owner for any part of
> the log.

Indeed, printing console input shifting should be a requirement
regardless of how we decide to print the prefix.

Thanks, Roger.