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

[dmkhn@xxxxxxxxx]

On Fri, Jun 06, 2025 at 09:12:06AM +0200, Jan Beulich wrote:
> On 06.06.2025 09:06, dmkhn@xxxxxxxxx wrote:
> > On Thu, Jun 05, 2025 at 08:18:34AM +0200, Jan Beulich wrote:
> >> On 05.06.2025 02:46, 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)".
> >>>
> >>> 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@xxxxxxxx>
> >>> ---
> >>> 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.
> 
> That is the host admin's decision, not a policy we should enforce.

re: policy: agreed, I will drop that hunk.

I think for the policy control, there can be a compile time setting (separate
patch) which enables/disables the debug output rate-limiting - and that setting
applies to:
  - vUARTs (currently vpl011 and "vuart", later ns16550 (x86) and upcoming
    emulator for RISC-V)
  - debug port on x86
  - HYPERVISOR_console_io

What do you think?

> 
> Jan