Re: [XEN][PATCH] console/consoleio: account for xen serial input focus during write

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.12.2025 00:32, Grygorii Strashko wrote:
> From: Grygorii Strashko <grygorii_strashko@xxxxxxxx>
> 
> When 2 or more domains are created and:
> - one is hwdom with "hvc0" (console_io) console
> - other DomUs with vpl011 or "hvc0" (console_io) console
> console output from hwdom may mix with output from other domains.
> 
> Example:
> [    2.288816] Key type id_legacy registered
> [    2.291894] n(XEN) DOM1: [    1.016950] DMA: preallocated 128 KiB 
> GFP_KERNEL|GFP_DMA32 pool for atomic allocations
> fs4filelayout_init: NFSv4 File Layout Driver Registering...
> (XEN) DOM1: [    1.018846] audit: initializing netlink subsys (disabled)
> 
> This happens because for hwdom the console output is produced by domain and
> handled by Xen as stream of chars, which can be interrupted when hwdom is
> scheduled out and so, cause console output mix.
> The Xen consoleio code trying to mimic serial HW behavior for hwdom
> unconditionally by sending available data to serial HW on arrival.
> Xen consoleio code does not account for Xen console input focus, comparing
> to emulated serial hw, like vpl011, which does the same for domain with
> active Xen console input focus only.
> 
> Switching console input focus to Xen improves situation, but not enough.
> 
> This patch changes consoleio code to account for domain with active Xen
> console input focus - console output will be sent directly to serial HW
> only if domain has active Xen console input focus. For other domains -
> console output will be buffered and sync on per-line basis.
> 
> Example output:
> (d2) [    4.263417] Key type id_legacy registered
> (XEN) DOM1: [    4.658080] Advanced Linux Sound Architecture Driver 
> Initialized.
> (d2) [    4.277824] nfs4filelayout_init: NFSv4 File Layout Driver 
> Registering...
> 
> Signed-off-by: Grygorii Strashko <grygorii_strashko@xxxxxxxx>
> ---
> This causes random multi-domain tests failures due to inter-domain console
> mixing which breaks console parsing checks.

This remark reads as if "here's a patch, but it breaks things". Instead of
merely "This", did you maybe mean "The original behavior" or some such?

Jan