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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.06.2025 22:11, dmkhn@xxxxxxxxx wrote:
> From: Denis Mukhin <dmukhin@xxxxxxxx>
> 
> 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.

Jan