Re: [PATCH] x86/console: process softirqs between warning prints

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.02.2022 09:28, Roger Pau Monne wrote:
> Process softirqs while printing end of boot warnings. Each warning can
> be several lines long, and on slow consoles printing multiple ones
> without processing softirqs can result in the watchdog triggering:
> 
> (XEN) [   22.277806] ***************************************************
> (XEN) [   22.417802] WARNING: CONSOLE OUTPUT IS SYNCHRONOUS
> (XEN) [   22.556029] This option is intended to aid debugging of Xen by 
> ensuring
> (XEN) [   22.696802] that all output is synchronously delivered on the serial 
> line.
> (XEN) [   22.838024] However it can introduce SIGNIFICANT latencies and affect
> (XEN) [   22.978710] timekeeping. It is NOT recommended for production use!
> (XEN) [   23.119066] ***************************************************
> (XEN) [   23.258865] Booted on L1TF-vulnerable hardware with 
> SMT/Hyperthreading
> (XEN) [   23.399560] enabled.  Please assess your configuration and choose an
> (XEN) [   23.539925] explicit 'smt=<bool>' setting.  See XSA-273.
> (XEN) [   23.678860] ***************************************************
> (XEN) [   23.818492] Booted on MLPDS/MFBDS-vulnerable hardware with 
> SMT/Hyperthreading
> (XEN) [   23.959811] enabled.  Mitigations will not be fully effective.  
> Please
> (XEN) [   24.100396] choose an explicit smt=<bool> setting.  See XSA-297.
> (XEN) [   24.240254] *************************************************(XEN) [ 
>   24.247302] Watchdog timer detects that CPU0 is stuck!
> (XEN) [   24.386785] ----[ Xen-4.17-unstable  x86_64  debug=y  Tainted:   C   
>  ]----
> (XEN) [   24.527874] CPU:    0
> (XEN) [   24.662422] RIP:    e008:[<ffff82d04025b84a>] 
> drivers/char/ns16550.c#ns16550_tx_ready+0x3a/0x90
> 
> Fixes: ee3fd57acd ('xen: add warning infrastructure')
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
>  xen/common/warning.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/xen/common/warning.c b/xen/common/warning.c
> index 0269c6715c..e6e1404baf 100644
> --- a/xen/common/warning.c
> +++ b/xen/common/warning.c
> @@ -30,6 +30,7 @@ void __init warning_print(void)
>      {
>          printk("%s", warnings[i]);
>          printk("***************************************************\n");
> +        process_pending_softirqs();
>      }

To be honest, I'm not convinced. This gets us pretty close to needing
to process softirqs after _every_ line of output. If a console is this
slow, the problem imo needs dealing with there (and according to irc
we appear on a helpful track there already), not by sprinkling more
process_pending_softirqs() all over the code.

Jan