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.stabell...@amd.com> > --- > 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
