Re: [PATCH v2 2/2] xen/console: unify printout behavior for UART emulators

[dmkhn@proton.me]

On Thu, Jun 05, 2025 at 08:18:34AM +0200, Jan Beulich wrote:
> On 05.06.2025 02:46, 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)".
> >
> > Use guest_printk() without rate-limiting in all current in-hypervisor UART
> > emulators. That aligns the behavior with debug I/O port 0xe9 handler on x86 and
> > slightly improves the logging since guest_printk() already prints the domain
> > ID. guest_printk() was modified to account for console focus ownership.
> >
> > Modify guest_console_write() for hardware domain case by adding domain ID to
> > the message when hwdom does not have console focus.
> >
> > [1] https://lore.kernel.org/xen-devel/alpine.DEB.2.22.394.2412121655360.463523@ubuntu-linux-20-04-desktop/
> > Signed-off-by: Denis Mukhin <dmukhin@ford.com>
> > ---
> > Changes since v1:
> > - dropped change for debug port and for HYPERVISOR_console_io hypercall
> 
> Yet then what about ...
> 
> > --- a/xen/arch/arm/vuart.c
> > +++ b/xen/arch/arm/vuart.c
> > @@ -89,7 +89,7 @@ static void vuart_print_char(struct vcpu *v, char c)
> >          if ( c != '\n' )
> >              uart->buf[uart->idx++] = '\n';
> >          uart->buf[uart->idx] = '\0';
> > -        printk(XENLOG_G_DEBUG "DOM%u: %s", d->domain_id, uart->buf);
> > +        guest_printk(d, "%s", uart->buf);
> >          uart->idx = 0;
> >      }
> >      spin_unlock(&uart->lock);
> 
> ... this dropping of XENLOG_G_DEBUG? In fact I'd have expected such to
> be _added_ where presently missing.

vUART is a debugging facility. This flavor of UART is specifically for guest OS
early boot debugging.
I think it is not desirable to potentially lose guest messages while doing such
early guest OS boot debugging.

> 
> Jan