Re: [PATCH v1 13/15] xen/riscv: implement reprogram_timer() using SBI

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.01.2026 13:41, Oleksii Kurochko wrote:
> On 1/14/26 12:17 PM, Jan Beulich wrote:
>> On 14.01.2026 11:33, Oleksii Kurochko wrote:
>>> On the other hand, if some
>>> other negative error code is returned, it might be better to return 0 and 
>>> simply
>>> allow the timer programming to be retried later.
>>> However, if we look at the comments for other architectures, the meaning of 
>>> a
>>> return value of 0 from this function is:
>>>    Returns 1 on success; 0 if the timeout is too soon or is in the past.
>>> In that case, it becomes difficult to distinguish whether 0 was returned 
>>> due to
>>> an error or because the timeout was too soon or already in the past.
>> Well, your problem is that neither Arm nor x86 can actually fail. Hence
>> calling code isn't presently prepared for that. With panic() (and hence
>> also BUG()) and domain_crash() ruled out, maybe generic infrastructure
>> needs touching first (in a different way than making the function's return
>> type "bool")?
> 
> I think making the function's return still is fine and it is only question to
> arch-specific reprogram_timer() what to do when an error happens.
> 
> Still doesn't clear to me what should be a reaction on failure of
> reprogram_timer().
> Considering that SBI spec doesn't specify a list of possible errors and now
> the only possible error is -ENOSUPP it seems to me it is fine
> to have panic() as we don't have any other mechanism to set a timer
> except SBI call

panic() (or BUG_ON()) is pretty drastic a measure when possibly the system
could be kept alive. If is pretty certain that future SBI timer calls also
aren't going to work, then I'd agree that panic()ing might be appropriate.
If otoh a subsequent call might work, a less heavyweight action would seem
preferable. (Welcome to the funs of relying on lower-level software.)

> (except the case SSTC is supported then we can use just
> supervisor timer register directly without SBI call).

So maybe a good first step would be to use that extension if available?
Might even think about requiring it for the time being ...

Jan