Re: [Qemu-devel] Re: [Bug 599958] Re: Timedrift problems with Win7: hpet missing time drift fixups

[Jan Kiszka <jan.kiszka@siemens.com>]

Avi Kivity wrote:
> On 07/05/2010 03:16 PM, Jan Kiszka wrote:
>>> It's also architecturally cleaner.  Masks and acks are architectural
>>> events.  Injections are not - there's the edge on the LINT0 or INTI2
>>> pins, generation of an APIC message, receipt of the APIC message, and
>>> assertion of the APIC-to-core interrupt interface.  I'm not sure how the
>>> proposed interface maps to that.
>>>      
>> Our emulation does not reflect every architectural detail of the
>> delivery path anyway.
> 
> Usually, when that happens, we get an obscure bug.
> 
> So if we add a facility, especially across the user/kernel boundary, 
> it's better to have it conform to the architecture.  That reduces the 
> chance it has a serious hidden bug.

Neither ack/mask notifiers (past the IRQ controller) nor injection
return values are part of any architecture we emulate. What is driving
us are the requirements of the de-coalescing workarounds we want to
build on top and the impact on existing design.

> 
>> The abstraction is always an IRQ line which can be
>> high or low (sometimes it is only high, but this is a bug).
>>    
> 
> That's a bug in the use of qemu_irq, not qemu_irq itself.
> 
> But qemu_irq needs to remember its state, otherwise when an irq 
> controller unmasks a level-triggered line, it won't see the interrupt.

I think it's currently the IRQ controller's job to keep track of the
line state during masked periods. Moving this to qemu_irq is definitely
better but requires some care (e.g. when vmstates are involved).

Jan

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