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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.07.2025 02:07, 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. Remove the unnecessary
> IPI on all physical CPUs to sync the TSC when the TSC is known to be
> reliable.

If it had been truly unnecessary for all the time, I'm sure someone would
have suggested to get rid of the overhead. IOW I think there is more to be
said as to this being correct / safe, including in any corner cases.

> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>
> ---
>  xen/arch/x86/time.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
> index 59129f419d..bfd022174a 100644
> --- a/xen/arch/x86/time.c
> +++ b/xen/arch/x86/time.c
> @@ -2303,6 +2303,10 @@ static void cf_check time_calibration(void *unused)
>          local_irq_enable();
>      }
>  
> +    if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
> +         boot_cpu_has(X86_FEATURE_TSC_RELIABLE) )
> +        return;

This would render the (first of the two) invocation(s) of the function from
verify_tsc_reliability() (largely) dead; it would then be only r.master_stime
which gets updated (see also Alejandro's reply), which surely wouldn't have
required that call in the first place.

Jan