Re: [PATCH RFC v3 1/3] time: add "NOW() good" indicator

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 5/20/26 4:45 PM, Jan Beulich wrote:
> printk_start_of_line() checks for a value of 0 right now. In order to be
> able to have NOW() return at least monotonically increasing values, that
> needs replacing by an explicit indicator.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> Arm and RISC-V may want to consider whether their initial get_cycles()
> can't be moved yet earlier, such that the indicator also can be set
> yet earlier.
> ---
At least, for RISC-V ...

> v3: New.
>
> --- a/xen/arch/arm/time.c
> +++ b/xen/arch/arm/time.c
> @@ -145,6 +145,7 @@ void __init preinit_xen_time(void)
>           panic("Timer: Cannot initialize platform timer\n");
>   
>       boot_count = get_cycles();
> +    NOW_good = true;
>   }
>   
>   static void __init init_dt_xen_time(void)
> --- a/xen/arch/riscv/time.c
> +++ b/xen/arch/riscv/time.c
> @@ -87,6 +87,7 @@ void __init preinit_xen_time(void)
>           panic("%s: ACPI isn't supported\n", __func__);
>   
>       boot_clock_cycles = get_cycles();
> +    NOW_good = true;
...  preinit_xen_time() call could be moved a little bit above just 
after riscv_fill_hwcap() as it is using riscv_isa_extension_available() 
inside.
preinit_xen_time() could be splited so it can be moved just above 
tasklet_subsys_init() after device tree is initialized but I don't think
there is to much sense for that.

>   
>       /* set_xen_timer must have been set by sbi_init() already */
>       ASSERT(set_xen_timer);
The ASSERT is harmless, but NOW_good could technically go after it.

boot_clock_cycles and cpu_khz must be visible to any reader of NOW_good 
== true. There's no explicit ordering between those stores and the 
NOW_good = true store. In practice this is fine because SMP isn't active 
at this point, but a WRITE_ONCE(NOW_good, true) or a compiler barrier 
would make the intent explicit and protect against future reordering by 
an optimizing compiler.
Thanks.

~ Oleksii