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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan