Re: [PATCH 2/2] xen/x86: ioapic: Bail out from timer_irq_works() as soon as possible

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.09.2023 16:00, Julien Grall wrote:
> Hi Jan,
> 
> On 07/09/2023 15:28, Jan Beulich wrote:
>> On 18.08.2023 15:44, Julien Grall wrote:
>>> From: Julien Grall <jgrall@xxxxxxxxxx>
>>>
>>> Currently timer_irq_works() will wait the full 100ms before checking
>>> that pit0_ticks has been incremented at least 4 times.
>>>
>>> However, the bulk of the BIOS/platform should not have a buggy timer.
>>> So waiting for the full 100ms is a bit harsh.
>>>
>>> Rework the logic to only wait until 100ms passed or we saw more than
>>> 4 ticks. So now, in the good case, this will reduce the wait time
>>> to ~50ms.
>>>
>>> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
>>
>> In principle this is all fine. There's a secondary aspect though which
>> may call for a slight rework of the patch.
>>
>>> --- a/xen/arch/x86/io_apic.c
>>> +++ b/xen/arch/x86/io_apic.c
>>> @@ -1509,6 +1509,8 @@ static void __init setup_ioapic_ids_from_mpc(void)
>>>   static int __init timer_irq_works(void)
>>>   {
>>>       unsigned long t1, flags;
>>> +    /* Wait for maximum 10 ticks */
>>> +    unsigned long msec = (10 * 1000) / HZ;
>>
>> (Minor remark: I don't think this needs to be unsigned long; unsigned
>> in will suffice.)
> 
> You are right. I can switch to unsigned int.
> 
>>
>>> @@ -1517,19 +1519,25 @@ static int __init timer_irq_works(void)
>>>   
>>>       local_save_flags(flags);
>>>       local_irq_enable();
>>> -    /* Let ten ticks pass... */
>>> -    mdelay((10 * 1000) / HZ);
>>> -    local_irq_restore(flags);
>>>   
>>> -    /*
>>> -     * Expect a few ticks at least, to be sure some possible
>>> -     * glue logic does not lock up after one or two first
>>> -     * ticks in a non-ExtINT mode.  Also the local APIC
>>> -     * might have cached one ExtINT interrupt.  Finally, at
>>> -     * least one tick may be lost due to delays.
>>> -     */
>>> -    if ( (ACCESS_ONCE(pit0_ticks) - t1) > 4 )
>>> +    while ( msec-- )
>>> +    {
>>> +        mdelay(1);
>>> +        /*
>>> +         * Expect a few ticks at least, to be sure some possible
>>> +         * glue logic does not lock up after one or two first
>>> +         * ticks in a non-ExtINT mode.  Also the local APIC
>>> +         * might have cached one ExtINT interrupt.  Finally, at
>>> +         * least one tick may be lost due to delays.
>>> +         */
>>> +        if ( (ACCESS_ONCE(pit0_ticks) - t1) <= 4 )
>>> +            continue;
>>> +
>>> +        local_irq_restore(flags);
>>>           return 1;
>>> +    }
>>> +
>>> +    local_irq_restore(flags);
>>>   
>>>       return 0;
>>>   }
>>
>> While Andrew has a patch pending (not sure why it didn't go in yet)
>> to simplify local_irq_restore(), and while further it shouldn't really
>> need using here (local_irq_disable() ought to be fine)
> 
> Skimming through the code, the last call of timer_irq_works() in 
> check_timer() happens after the interrupts masking state have been restored:
> 
> local_irq_restore(flags);
> 
> if ( timer_irq_works() )
>    ...
> 
> 
> So I think timer_irq_works() can be called with interrupts enabled and 
> therefore we can't use local_irq_disable().

Hmm, yes, you're right. That's inconsistent, but dealing with that is a
separate task.

Jan