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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Fri, Jun 06, 2025 at 08:11:26PM +0000, 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() 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 v2:
> - dropped rate-limiting change for vuart
> ---
>  xen/arch/arm/vpl011.c      |  6 +++---
>  xen/arch/arm/vuart.c       |  2 +-
>  xen/drivers/char/console.c | 23 +++++++++++++++++++++--
>  3 files changed, 25 insertions(+), 6 deletions(-)
> 
> diff --git a/xen/arch/arm/vpl011.c b/xen/arch/arm/vpl011.c
> index 480fc664fc..2b6f2a09bc 100644
> --- a/xen/arch/arm/vpl011.c
> +++ b/xen/arch/arm/vpl011.c
> @@ -87,7 +87,7 @@ static void vpl011_write_data_xen(struct domain *d, uint8_t data)
>      {
>          if ( intf->out_prod == 1 )
>          {
> -            printk("%c", data);
> +            guest_printk(d, "%c", data);
>              intf->out_prod = 0;
>          }
>          else
> @@ -95,7 +95,7 @@ static void vpl011_write_data_xen(struct domain *d, uint8_t data)
>              if ( data != '\n' )
>                  intf->out[intf->out_prod++] = '\n';
>              intf->out[intf->out_prod++] = '\0';
> -            printk("%s", intf->out);
> +            guest_printk(d, "%s", intf->out);
>              intf->out_prod = 0;
>          }
>      }
> @@ -107,7 +107,7 @@ static void vpl011_write_data_xen(struct domain *d, uint8_t data)
>              if ( data != '\n' )
>                  intf->out[intf->out_prod++] = '\n';
>              intf->out[intf->out_prod++] = '\0';
> -            printk("DOM%u: %s", d->domain_id, intf->out);
> +            guest_printk(d, "%s", intf->out);
>              intf->out_prod = 0;
>          }
>      }
> diff --git a/xen/arch/arm/vuart.c b/xen/arch/arm/vuart.c
> index bd2f425214..6641f9d775 100644
> --- 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, XENLOG_G_DEBUG "%s", uart->buf);
>          uart->idx = 0;
>      }
>      spin_unlock(&uart->lock);
> diff --git a/xen/drivers/char/console.c b/xen/drivers/char/console.c
> index 6e77b4af82..3855962af7 100644
> --- 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);
>          }
> @@ -1032,12 +1042,21 @@ void printk(const char *fmt, ...)
>      va_end(args);
>  }
>  
> +/*
> + * Print message from the guest on the diagnostic console.
> + * Prefixes all messages w/ "(dX)" if domain X does not own physical console
> + * focus.
> + */
>  void guest_printk(const struct domain *d, const char *fmt, ...)
>  {
>      va_list args;
> -    char prefix[16];
> +    char prefix[16] = "";
> +    struct domain *consd;
>  
> -    snprintf(prefix, sizeof(prefix), "(d%d) ", d->domain_id);
> +    consd = console_get_domain();
> +    if ( consd != d )
> +        snprintf(prefix, sizeof(prefix), "(d%d) ", d->domain_id);
> +    console_put_domain(consd);

It might be helpful to abstract this into a separate helper, as it's
used by both functions:

static void fill_console_prefix(char *prefix, unsigned int len,
                                const struct domain *d)
{
    struct domain *consd = console_get_domain();

    if ( consd ? consd != d : !is_hardware_domain(d)) )
       snprintf(prefix, len, "(d%d) ", d->domain_id);
    console_put_domain(consd);
}

Note the above code should also handle the current discussion of not
printing the (d0) prefix for the hardware domain when the console
target is Xen.  I think this keeps the previous behavior when console
input is switched to Xen, while still providing unified (dX) prefixes
otherwise.

Thanks, Roger.