Re: [PATCH v2] xen: Expose time_offset in struct arch_shared_info

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.01.2026 10:57, Tu Dinh wrote:
> time_offset is currently always added to wc_sec. This means that without
> the actual value of time_offset, guests have no way of knowing what's
> the actual host clock. Once the guest clock drifts beyond 1 second,
> updates to the guest RTC would themselves change time_offset and make it
> impossible to resync guest time to host time.

Despite my earlier comments this part of the description looks unchanged.
I still don't see why host time (or in fact about any host property) should
be exposed to guests.

> Since there's no way to add more fields to struct shared_info, the
> addition has to be done through struct arch_shared_info instead. Add two
> fields in arch_shared_info representing time_offset's low and high
> 32-bit halves.

Again, despite my earlier question, reasoning of why two halves rather than
a (signed) 64-bit value isn't supplied here.

> Provide a new feature bit XENFEAT_shared_info_time_offset for this
> functionality.
> 
> Signed-off-by: Tu Dinh <ngoc-tu.dinh@xxxxxxxxxx>
> ---
> v2: Remove unnecessary casts.

Did you really? What about ...

> --- a/xen/common/time.c
> +++ b/xen/common/time.c
> @@ -118,6 +118,11 @@ void update_domain_wallclock_time(struct domain *d)
>      shared_info(d, wc_sec_hi) = sec >> 32;
>  #endif
>  
> +    shared_info(d, arch.time_offset) =
> +        (uint32_t)d->time_offset.seconds;
> +    shared_info(d, arch.time_offset_hi) =
> +        (uint32_t)(d->time_offset.seconds >> 32);

... both of these?

Jan