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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.01.2026 10:41, Oleksii Kurochko wrote:
> 
> On 1/14/26 10:13 AM, Jan Beulich wrote:
>> On 13.01.2026 17:50, Oleksii Kurochko wrote:
>>> On 1/12/26 4:24 PM, Jan Beulich wrote:
>>>> On 24.12.2025 18:03, Oleksii Kurochko wrote:
>>>>> @@ -39,6 +43,33 @@ static void __init preinit_dt_xen_time(void)
>>>>>        cpu_khz = rate / 1000;
>>>>>    }
>>>>>    
>>>>> +int reprogram_timer(s_time_t timeout)
>>>>> +{
>>>>> +    uint64_t deadline, now;
>>>>> +    int rc;
>>>>> +
>>>>> +    if ( timeout == 0 )
>>>>> +    {
>>>>> +        /* Disable timers */
>>>>> +        csr_clear(CSR_SIE, BIT(IRQ_S_TIMER, UL));
>>>>> +
>>>>> +        return 1;
>>>>> +    }
>>>>> +
>>>>> +    deadline = ns_to_ticks(timeout) + boot_clock_cycles;
>>>>> +    now = get_cycles();
>>>>> +    if ( deadline <= now )
>>>>> +        return 0;
>>>>> +
>>>>> +    /* Enable timer */
>>>>> +    csr_set(CSR_SIE, BIT(IRQ_S_TIMER, UL));
>>>> Still learning RISC-V, so question for my understanding: Even if the 
>>>> timeout
>>>> is short enough to expire before the one SIE bit will be set, the interrupt
>>>> will still occur (effectively immediately)? (Else the bit may need setting
>>>> first.)
>>> The interrupt will become pending first (when mtime >= mtimecmp or
>>> mtime >= CSR_STIMECMP in case of SSTC) and then fire immediately once
>>> |SIE.STIE |(and global|SIE|) are enabled.
>>>
>>>>> +    if ( (rc = sbi_set_timer(deadline)) )
>>>>> +        panic("%s: timer wasn't set because: %d\n", __func__, rc);
>>>> Hmm, if this function ends up being used from any guest accessible path 
>>>> (e.g.
>>>> a hypercall), such panic()-ing better shouldn't be there.
>>> I don't have such use cases now and I don't expect that guest should use
>>> this function.
>> How do you envision supporting e.g. VCPUOP_set_singleshot_timer without
>> involving this function?
> 
> Looking at what is in common code for VCPUOP_set_singleshot_timer, it doesn't
> use reprogram_timer(), it is just activate/deactivate timer.

And how would that work without, eventually, using reprogram_timer()? While not
directly on a hypercall path, the use can still be guest-induced.

Jan