Re: [PATCH v4 1/3] time: add "NOW() good" indicator

[Roger Pau Monné <roger@xxxxxxxxxxxxxx>]

On Tue, Jul 28, 2026 at 10:24:38AM +0200, Jan Beulich wrote:
> On 28.07.2026 10:17, Roger Pau Monné wrote:
> > On Tue, Jun 30, 2026 at 04:06:00PM +0200, Jan Beulich wrote:
> >> --- a/xen/arch/x86/time.c
> >> +++ b/xen/arch/x86/time.c
> >> @@ -2660,6 +2660,7 @@ void __init early_time_init(void)
> >>  
> >>      set_time_scale(&t->tsc_scale, tmp);
> >>      t->stamp.local_tsc = boot_tsc_stamp;
> >> +    NOW_good = true;
> > 
> > Would you need a barrier here to ensure compiler doesn't re-order the
> > writes?  Maybe using ACCESS_ONCE(), or a smp_wmb() like it's used in
> > other arches?
> 
> Aiui it isn't needed here. The compiler can't re-order the two writes,
> due to our use of -fno-strict-aliasing. The barrier is there on other
> arch-es to avoid re-ordering in hardware.

You are the expert in compilers, but it was my understanding that
`no-strict-aliasing` only affects the ordering of pointers accesses,
but not plain variables.  IOW: NOW_good accesses could be reordered
because it's not a pointer.  Does the compiler consider t can point to
&NOW_good and hence it can't be reordered?

Thanks, Roger.