Re: Need help on a issue (Unable to schedule guest for Xen on Arm)

[Ayan Kumar Halder <ayankuma@xxxxxxx>]

(Apologies, fixed the formatting issue)

On 30/03/2023 16:50, Ayan Kumar Halder wrote:
> Hi Xen experts,
>
> I need some pointers on an issue I am facing.
>
> I am running my downstream port of Xen on Cortex-R52 hardware. The 
> hardware consist of two R52 cores (the second core is in lockstep 
> mode). So, currently the hardware does not support SMP.
> The issue is that Xen is unable to schedule a guest. So 
> leave_hypervisor_to_guest() ---> check_for_pcpu_work() and this does 
> not return.
> Debugging this, I see  check_for_pcpu_work() --> do_softirq() --> 
> __do_softirq() --> timer_softirq_action().
> In timer_softirq_action(), the problem I see is that for all the 
> timers, "((t = heap[1])->expires < now)" is true.
>     while ( (heap_metadata(heap)->size != 0) &&
>             ((t = heap[1])->expires < now) )
>     {
>         remove_from_heap(heap, t); <<<<------ So, this gets invoked 
> for all the timers.
>         execute_timer(ts, t);
>     }
>
> So, further below reprogram_timer() gets invoked with timeout = 0 and 
> it disables the timer. So, timer_interrupt() is never invoked.
> Can someone give any pointers on what the underlying issue could be 
> and how to debug further ?
> I do not observe this behavior while running on R52 FVP. The 
> difference is that for most of the timers "((t = heap[1])->expires < 
> now)" is false, so reprogram_timer() gets invoked with non zero 
> timeout and subsequently, the timer_interrupt() get invoked.
> Also, looking at the following from xen/arch/arm/time.c.
/* Set the timer to wake us up at a particular time.
 * Timeout is a Xen system time (nanoseconds since boot); 0 disables 
the timer.
 * Returns 1 on success; 0 if the timeout is too soon or is in the past. */
int reprogram_timer(s_time_t timeout)
{
    uint64_t deadline;

    if ( timeout == 0 )
    {
        WRITE_SYSREG(0, CNTHP_CTL_EL2);
        return 1; <<<<<<<<<<<<<<<<<<<<<<-------------- Shouldn't this 
be 0 as the comment suggets ?
    }

    deadline = ns_to_ticks(timeout) + boot_count;
    WRITE_SYSREG64(deadline, CNTHP_CVAL_EL2);
    WRITE_SYSREG(CNTx_CTL_ENABLE, CNTHP_CTL_EL2);
    isb();

    /* No need to check for timers in the past; the Generic Timer fires
     * on a signed 63-bit comparison. */
    return 1;
}

> Kind regards,
>
> Ayan