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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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.

Hi Jan,

It looks like we are aligned on the desired behavior, 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

It is OK to change this behavior, but in that case I would ask that we
change it consistently also for ARM.