RE: [Xen-ia64-devel] Event channel vs current scheme speed [wasvIOSAPIC and IRQs delivery]

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

>From: Tristan Gingold [mailto:Tristan.Gingold@xxxxxxxx]
>Sent: 2006年3月13日 17:32
>
>Le Jeudi 09 Mars 2006 21:02, Tian, Kevin a écrit :
>> Anyway, good discussion by far though still some way to go for
>> consensus. :-)
>>
>> Maybe we want to look at this from another way - fairness.
>[...]
>> Regarding current model, there seems to be an issue about fairness
>> between physical interrupts and "xen events". Taking current 0xE9 for
>> example, it's lower than timer but higher than all external device
>> interrupts. This means "xen events" will always preempt device interrupts
>> in this case, which is unfair and not what we want.
>To my understanding, this is also true for x86.
>With event channel, real physical IRQs use events 0-255, while Xen events
>use
>events 256-511.
>
>So what is the difference ?
>

Difference is obvious, because 0-255 or 256-511 is not the first level of 
priority decision. The base line is always evtchn_pending, with lower bit 
for higher priority by far. Phys_irq may have event port higher than the 
one owned by a dyn_irq, thus the priority of the former is instead lower 
than the latter. Phys_irq in 0-255 and dyn_irq in 256-511 are just one 
compatible way to the end user, with former indicating normal interrupt 
while the latter for new type of events.

Yes, currently the priority of event channel is a simple way as 
earlier-come-higher-priority, which is decided by the init sequence of 
different drivers. But once event layer is added under traditional interrupt, 
you can choose more complex/accurate priority policy on demand. 
That's it!

Thanks,
Kevin

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