Re: [Qemu-devel] Re: [RFT][PATCH 07/15] qemu_irq: Add IRQ handlers with delivery feedback

[Jan Kiszka <jan.kiszka@web.de>]

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Blue Swirl wrote:
> On Wed, May 26, 2010 at 11:26 PM, Paul Brook <paul@codesourcery.com> wrote:
>>> At the other extreme, would it be possible to make the educated guests
>>> aware of the virtualization also in clock aspect: virtio-clock?
>> The guest doesn't even need to be aware of virtualization. It just needs to be
>> able to accommodate the lack of guaranteed realtime behavior.
>>
>> The fundamental problem here is that some guest operating systems assume that
>> the hardware provides certain realtime guarantees with respect to execution of
>> interrupt handlers.  In particular they assume that the CPU will always be
>> able to complete execution of the timer IRQ handler before the periodic timer
>> triggers again.  In most virtualized environments you have absolutely no
>> guarantee of realtime response.
>>
>> With Linux guests this was solved a long time ago by the introduction of
>> tickless kernels.  These separate the timekeeping from wakeup events, so it
>> doesn't matter if several wakeup triggers end up getting merged (either at the
>> hardware level or via top/bottom half guest IRQ handlers).
>>
>>
>> It's worth mentioning that this problem also occurs on real hardware,
>> typically due to lame hardware/drivers which end up masking interrupts or
>> otherwise stall the CPU for for long periods of time.
>>
>>
>> The PIT hack attempts to workaround broken guests by adding artificial latency
>> to the timer event, ensuring that the guest "sees" them all.  Unfortunately
>> guests vary on when it is safe for them to see the next timer event, and
>> trying to observe this behavior involves potentially harmful heuristics and
>> collusion between unrelated devices (e.g. interrupt controller and timer).
>>
>> In some cases we don't even do that, and just reschedule the event some
>> arbitrarily small amount of time later. This assumes the guest to do useful
>> work in that time. In a single threaded environment this is probably true -
>> qemu got enough CPU to inject the first interrupt, so will probably manage to
>> execute some guest code before the end of its timeslice. In an environment
>> where interrupt processing/delivery and execution of the guest code happen in
>> different threads this becomes increasingly likely to fail.
> 
> So any voodoo around timer events is doomed to fail in some cases.
> What's the amount of hacks what we want then? Is there any generic

The aim of this patch is to reduce the amount of existing and upcoming
hacks. It may still require some refinements, but I think we haven't
found any smarter approach yet that fits existing use cases.

> solution, like slowing down the guest system to the point where we can
> guarantee the interrupt rate vs. CPU execution speed?

That's generally a non-option in virtualized production environments.
Specifically if the guest system lost interrupts due to host
overcommitment, you do not want it slow down even further.

Jan


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