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

[Paul Brook]

> 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.

Paul