Re: [PATCH v10 5/5] xen: enable dom0less guests to use console_io hypercalls

[dmukhin@ford.com]

On Thu, Feb 05, 2026 at 02:09:07PM -0800, Stefano Stabellini wrote:
> On Wed, 4 Feb 2026, dmukhin@xen.org wrote:
> > On Wed, Feb 04, 2026 at 03:37:12PM -0800, Stefano Stabellini wrote:
> > > Enable dom0less guests on ARM to use console_io hypercalls:
> > > - set input_allow = true for dom0less domains
> > > - update the in-code comment in console.c
> > > - prioritize the VUART check to retain the same behavior as today
> > > 
> > > Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
> > 
> > The code looks good, just one remark wrt prioritizing VUART check.
> > 
> > > ---
> > >  xen/common/device-tree/dom0less-build.c |  2 ++
> > >  xen/drivers/char/console.c              | 16 ++++++++++------
> > >  2 files changed, 12 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/xen/common/device-tree/dom0less-build.c b/xen/common/device-tree/dom0less-build.c
> > > index 840d14419d..cb7026fa7e 100644
> > > --- a/xen/common/device-tree/dom0less-build.c
> > > +++ b/xen/common/device-tree/dom0less-build.c
> > > @@ -829,6 +829,8 @@ static int __init construct_domU(struct kernel_info *kinfo,
> > >  
> > >      rangeset_destroy(kinfo->xen_reg_assigned);
> > >  
> > > +    d->console->input_allowed = true;
> > > +
> > >      return rc;
> > >  }
> > >  
> > > diff --git a/xen/drivers/char/console.c b/xen/drivers/char/console.c
> > > index d3ce925131..7f0c3d8376 100644
> > > --- a/xen/drivers/char/console.c
> > > +++ b/xen/drivers/char/console.c
> > > @@ -610,11 +610,20 @@ static void __serial_rx(char c)
> > >      if ( ACCESS_ONCE(console_rx) == 0 )
> > >          return handle_keypress(c, false);
> > >  
> > > +    /* Includes an is_focus_domain() check. */
> > >      d = console_get_domain();
> > >      if ( !d )
> > >          return;
> > >  
> > > -    if ( is_hardware_domain(d) )
> > 
> > Hardware domain on x86 may have an emulated UART (not in upstream, through,
> > I need to send v8 for NS16550 series...). The patch which illustrates the
> > idea:
> >    https://lore.kernel.org/xen-devel/20250908211149.279143-2-dmukhin@ford.com/
> > 
> > So this code (hopefully soon) will need adjustment again.
> >
> > I would update the code to something like:
> > 
> > 
> > 
> >     if ( is_hardware_domain(d) && !domain_has_vuart(d) )
> >     {
> >         // handle hardware domain
> >     }
> > #ifdef CONFIG_SBSA_VUART_CONSOLE
> >     else if ( domain_has_vuart(d) )
> >         /* Deliver input to the emulated UART. */
> >         rc = vpl011_rx_char_xen(d, c);
> > #endif
> > 
> > 
> > 
> > But domain_has_vuart() needs to be defined for all architectures
> > (currently it is hidden in arch/arm/vuart.c).
> > 
> > Or perhaps it is possible to postpone the change?
> 
> This change is needed to avoid regressions on ARM.

Oh, I see.

> 
> However, while I wouldn't be surprised if we need a change here for your
> upcoming patch series, at the same time at the moment I don't see why
> this check wouldn't work as it is for you as well.
> 
> On x86, CONFIG_SBSA_VUART_CONSOLE will never be set. It is OK to do this
> first:
> 
> #ifdef CONFIG_SBSA_VUART_CONSOLE
>     /* Prioritize vpl011 if enabled for this domain */
>     if ( d->arch.vpl011.base_addr )
>     {
>         /* Deliver input to the emulated UART. */
>         rc = vpl011_rx_char_xen(d, c);
>     }
>     else
> #endif
> 
> It shouldn't hurt. The is_hardware_domain() check is also not necessary
> anymore because any necessary check would be part of this check above: 
> 
>       d = console_get_domain();
>       if ( !d )
>           return;
> 
> So I am guessing that your series might actually leave this code
> unchanged and instead might modify console_get_domain() or
> max_console_rx.

I think I did experiment w/ processing vUART before hardware domain
locally when debugging NS16550 emulator with dom0 PVH...

Thanks for explanations!

Please consider:

Reviewed-by: Denis Mukhin <dmukhin@ford.com> 

> 
> 
> 
> > > +#ifdef CONFIG_SBSA_VUART_CONSOLE
> > > +    /* Prioritize vpl011 if enabled for this domain */
> > > +    if ( d->arch.vpl011.base_addr )
> > > +    {
> > > +        /* Deliver input to the emulated UART. */
> > > +        rc = vpl011_rx_char_xen(d, c);
> > > +    }
> > > +    else
> > > +#endif
> > >      {
> > >          unsigned long flags;
> > >  
> > > @@ -633,11 +642,6 @@ static void __serial_rx(char c)
> > >           */
> > >          send_guest_domain_virq(d, VIRQ_CONSOLE);
> > >      }
> > > -#ifdef CONFIG_SBSA_VUART_CONSOLE
> > > -    else
> > > -        /* Deliver input to the emulated UART. */
> > > -        rc = vpl011_rx_char_xen(d, c);
> > > -#endif
> > >  
> > >      if ( consoled_is_enabled() )
> > >          /* Deliver input to the PV shim console. */
> > > -- 
> > > 2.25.1
> > > 
> > > 
> >