Re: [PATCH v3 2/2] xen/console: unify printout behavior for UART emulators

[dmkhn@proton.me]

On Tue, Jun 10, 2025 at 10:21:40AM +0200, Jan Beulich wrote:
> On 06.06.2025 22:11, dmkhn@proton.me wrote:
> > From: Denis Mukhin <dmukhin@ford.com>
> >
> > If virtual UART from domain X prints on the physical console, the behavior is
> > updated to (see [1]):
> > - console focus in domain X: do not prefix messages;
> > - no console focus in domain X: prefix all messages with "(dX)".
> 
> While, as indicated (much) earlier, I can see why omitting the prefix
> may make sense for the domain having input focus, ...
> 
> > --- a/xen/drivers/char/console.c
> > +++ b/xen/drivers/char/console.c
> > @@ -740,7 +740,17 @@ static long guest_console_write(XEN_GUEST_HANDLE_PARAM(char) buffer,
> >          if ( is_hardware_domain(cd) )
> >          {
> >              /* Use direct console output as it could be interactive */
> > +            char prefix[16] = "";
> > +            struct domain *consd;
> > +
> > +            consd = console_get_domain();
> > +            if ( consd != cd )
> > +                snprintf(prefix, sizeof(prefix), "(d%d) ", cd->domain_id);
> > +            console_put_domain(consd);
> > +
> >              nrspin_lock_irq(&console_lock);
> > +            if ( prefix[0] != '\0' )
> > +                console_send(prefix, strlen(prefix), flags);
> >              console_send(kbuf, kcount, flags);
> >              nrspin_unlock_irq(&console_lock);
> >          }
> 
> ... this, aiui, is a behavioral change for the non-dom0less case, where
> Dom0 output will suddenly also gain the prefix. Which I don't think is
> wanted: Switching focus between Xen and Dom0 should remain unaffected
> in this regard.

This change ensures that dom0 traces aren't mixed with domU traces when domU
has input focus, or with Xen traces when the administrator is in the diagnostic
console.

Also, this can be used in the non-interactive tests (e.g. XTF).

> 
> Jan
>