Re: [PATCH v2 1/2] xen/x86: don't send IPI to sync TSC when it is reliable

[Alejandro Vallejo <alejandro.garciavallejo@amd.com>]

On Tue Jul 8, 2025 at 8:32 PM CEST, Stefano Stabellini wrote:
> On real time configuration with the null scheduler, we shouldn't
> interrupt the guest execution unless strictly necessary: the guest could
> be a real time guest (e.g. FreeRTOS) and interrupting its execution
> could lead to a missed deadline. The principal source of interruptions
> is IPIs.
>
> When TSC is the chosen clocksource, we know it is reliable and
> synchronized across cpus and clusters. Thus, we can return early
> time_calibration because the calibration is not needed, removing the
> related Xen timer and IPIs.
>
> Also remove the master_stime write as it is unnecessary.
>
> Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
> ---
> Changes in v2:
> - simplify the patch simply by returning early if clocksource_is_tsc()
> - also remove setting r.master_stime as it is not needed
> ---
>  xen/arch/x86/time.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
>
> diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
> index 59129f419d..d72e640f72 100644
> --- a/xen/arch/x86/time.c
> +++ b/xen/arch/x86/time.c
> @@ -2297,11 +2297,7 @@ static void cf_check time_calibration(void *unused)
>      };
>  
>      if ( clocksource_is_tsc() )
> -    {
> -        local_irq_disable();
> -        r.master_stime = read_platform_stime(&r.master_tsc_stamp);
> -        local_irq_enable();
> -    }
> +        return;
>  
>      cpumask_copy(&r.cpu_calibration_map, &cpu_online_map);
>  

As far as I can tell, this shouldn't cause problems. But I'd prefer if someone
knowledgeable in the calibration code (Jan?) pitches in as to the effects of
master_stime in the absence of calibration. Otherwise:

  Reviewed-by: Alejandro Vallejo <alejandro.garciavallejo@amd.com>

Cheers,
Alejandro