Re: [Qemu-devel] [PATCH v7 1/2] hw/ptimer: Fix issues caused by artificially limited timer timeout

[Dmitry Osipenko <digetx@gmail.com>]

25.10.2015 02:55, Peter Crosthwaite пишет:
> On Sat, Oct 24, 2015 at 3:22 PM, Dmitry Osipenko <digetx@gmail.com> wrote:
>> 24.10.2015 22:45, Peter Crosthwaite пишет:
>>>
>>>
>>> This looks like a give-up without trying to get the correct value. If
>>> the calculated value (using the normal-path logic below) is sane, you
>>> should just use it. If it comes out bad then you should clamp to 1.
>>>
>>> I am wondering whether this clamping policy (as in original code as
>>> well) is correct at all though. The value of a free-running
>>> short-interval periodic timer (poor mans random number generator)
>>> without any actual interrupt generation will be affected by QEMUs
>>> asynchronous handling of timer events. So if I set limit to 100, then
>>> sample this timer every user keyboard stroke, I should get a uniform
>>> distribution on [0,100]. Instead in am going to get lots of 1s. This
>>
>>
>> Right, that's a good example. What about to scale ptimer period to match
>> adjusted timer_mod interval?
>>
>
> Thats just as incorrect as changing the limit IMO. The guest could get
> confused with a timer running at the wrong frequency.
>
>>> is more broken in the original code (as you state), as I will get >
>>> 100, but I think we have traded broken for slightly less broken. I
>>> think the correct semantic is to completely ignoring rate limitin
>>> except for the scheduling on event callbacks. That is, the timer
>>
>>
>> I'm missing you here. What event callbacks?
>>
>
> when timer_mod() hits, and it turn triggers some device specific event
> (usually an interrupt).
>
> There are two basic interactions for any QEMU timer. There are
> synchronous events, i.e. the guest reading (polling) the counter which
> is what this patch tries to fix. The second is the common case of
> periodic interrupt generation. My proposal is that rate limiting does
> not affect synchronous operation, only asynchronous (so my keystroke
> RNG case works). In the current code, if ptimer_get_count() is called
> when the event has passed it returns 0 under the assumption that the
> timer_mod callback is about to happen. With rate-limiting that may be
> well in the future.
>

ptimer_tick() would happen on the next QEMU loop cycle, so it might be more 
reasonable to return counter = 1 here, wouldn't it?

>>> interval is not rate limited, instead the timer_mod interval
>>> (next_event -last_event) just has a 10us clamp.
>>>
>>> The means the original code semantic of returning counter = 0 for an
>>> already triggered timer is wrong. It should handle in-the-past
>>> wrap-arounds as wraparounds by triggering the timer and redoing the
>>> math with the new interval values. So instead the logic would be
>>> something like:
>>>
>>> timer_val = -1;
>>>
>>> for(;;) {
>>>
>>>    if (!enabled) {
>>>       return delta;
>>>    }
>>>
>>>    timer_val = (next-event - now) *  scaling();
>>>    if (timer_val >=  0) {
>>>        return timer_val;
>>>    }
>>>    /* Timer has actually expired but we missed it, reload it and try again
>>> */
>>>    ptimer_tick();
>>> }
>>
>>
>> Why do you think that ptimer_get_count() == 0 in case of the running
>> periodic timer that was expired while QEMU was "on the way" to ptimer code
>> is bad and wrong?
>
> Because you may have gone past the time when it was actually zero and
> reloaded and started counting again. It should return the real
> (reloaded and resumed) value. This is made worse by rate limiting as
> the timer will spend a long time at the clamp value waiting for the
> rate-limited tick to fix it.
>
> Following on from before, we don't want the async stuff to affect
> sync. As the async callbacks are heavily affected by rate limiting we
> don't want the polled timer having to rely on the callbacks (async
> path) at all for correct operation. That's what the current
> implementation of ptimer_get_count assumes with that 0-clamp.
>

Alright, that make sense now. Thanks for clarifying.

>>  From the guest point of view it's okay (no?), do we really
>> need to overengineer that corner case?
>>
>
> Depends on your use case. Your bug report is correct in that the timer
> should never be outside the bounds of the limit. But you are fixing
> that very specifically with a minimal change rather than correcting
> the larger ptimer_get_count() logic which I think is more broken than
> you suggest it is.
>
>>>
>>> ptimer_reload() then needs to be patched to make sure it always
>>> timer_mod()s in the future, otherwise this loop could iterate a large
>>> number of times.
>>>
>>> This means that when the qemu_timer() actually ticks, a large number
>>> or cycles may have occured, but we can justify that in that callback
>>> event latency (usually interrupts) is undefined anyway and coalescing
>>> of multiples may have happened as part of that. This usually matches
>>> expectations of real guests where interrupt latency is ultimately
>>> undefined.
>>
>>
>> ptimer_tick() is re-arm'ing the qemu_timer() in case of periodic mode. Hope
>> I haven't missed your point here.
>>
>
> Yes. But it is also capable of doing the heavy lifting for our already
> expired case. Basic idea is, if the timer is in a bad state (should
> have hit) but hasn't, do the hit to put the timer into a good state
> (by calling ptimer_tick) then just do the right thing. That's what the
> loop does. It should also work for an in-the-past one-shot.
>

Summarizing what we have now:

There are two issues with ptimer:

1) ptimer_get_count() return wrong values with adjusted .limit

Patch V7 doesn't solve that issue, but makes it slightly better by clamping 
returned value to [0, 1]. That's not what we want, we need to return counter 
value in it's valid range [0, limit].

You are rejecting variant of scaling ptimer period, saying that it might affect 
software behavior inside the guest. But by adjusting the timer, we might already 
causing same misbehavior in case of blazing fast host machine. I'll scratch my 
head a bit more on it. If you have any better idea, please share.

2) ptimer_get_count() return fake 0 value in case of the expired qemu_timer() 
without triggering ptimer_tick()

You're suggesting to solve it by running ptimer_tick(). So if emulated device 
uses ptimer tick event (scheduled qemu bh) to raise interrupt, it would do it by 
one QEMU loop cycle earlier.

My question here: is it always legal for the guest software to be able to get 
counter = 0 on poll while CPU interrupt on timer expire hasn't happened yet 
(would happen after one QEMU cycle). I guess it might cause software misbehavior 
if it assumes that the real hardware has CPU and timer running in the same clock 
domain, i.e. such situation might be not possible. So I'm suggesting to return 
counter = 1 and allow ptimer_tick() happen on it's own.

-- 
Dmitry