Re: [PATCH v3 13/16] xen/riscv: implement reprogram_timer() via SBI

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.02.2026 17:52, Oleksii Kurochko wrote:
> Implement reprogram_timer() on RISC-V using the standard SBI timer call.
> 
> The privileged architecture only defines machine-mode timer interrupts
> (using mtime/mtimecmp). Therefore, timer services for S/HS/VS mode must
> be provided by M-mode via SBI calls. SSTC (Supervisor-mode Timer Control)
> is optional and is not supported on the boards available to me, so the
> only viable approach today is to program the timer through SBI.
> 
> reprogram_timer() enables/disables the supervisor timer interrupt and
> programs the next timer deadline using sbi_set_timer(). If the SBI call
> fails, the code panics, because sbi_set_timer() is expected to return
> either 0 or -ENOSUPP (this has been stable from early OpenSBI versions to
> the latest ones). The SBI spec does not define a standard negative error
> code for this call, and without SSTC there is no alternative method to
> program the timer, so the SBI timer call must be available.
> 
> reprogram_timer() currently returns int for compatibility with the
> existing prototype. While it might be cleaner to return bool, keeping the
> existing signature avoids premature changes in case sbi_set_timer() ever
> needs to return other values (based on which we could try to avoid
> panic-ing) in the future.
> 
> Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

> ---
> Changes in v3:
>  - Correct the comments in reprogram_timer().
>  - Move enablement of timer interrupt after sbi_set_timer() to avoid
>    potentially receiving a timer interrupt between these 2 operations.

I'd like to mention that this is of only hypothetical concern, at least for
the sole caller in common code: That's doing the call with IRQs off, so
only the bit in SIP could become set too early, while no IRQ would surface
before timer_softirq_action() turns IRQs on again. (This isn't to say that
it wasn't a good thing to adjust the order.)

Jan