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

["Dong, Eddie" <eddie.dong@xxxxxxxxx>]

Tristan Gingold wrote:
> Sorry, I was not clear enough.  I agree event channel can mostly
> respect priority.

That is OK, everybody may forget something or ignore something.

> 
> Except clock and IPI, I think Linux doesn't use priority.  I can't
> force a card to have an higher priority than another card.
> Is it right ?
No. Linux well handle this.
In Itanium architecture, IRQ priority is generated in LSPAIC by vector 
number, see SDM2 Table 5-8. In summary, software can program
each RTE on IOSAPIC with certain vector #.  High vector # usually 
means high priority (plus class inf). Then in linux software,
ia64_handle_irq() do like following:
   while (IRQ exist) {
        vector=CR.IVR;
        mask TPR for lower than vector;
        do_IRQ();
        Issue CR.EOI
   }

   Here read IVR get highest priority IRQ and mask lower IRQ using TPR. 
Within do_IRQ(), if a IRQ higher than previous one happens, processor is

interrupted again and reenter  ia64_handle_irq(). So everything turns
fine :-)

    Actually clock and IPI is also handled in this mechanism, they are
equal 
with other IRQs except the vector #.  So you can assign a card with
higher
IRQ # such as for ethernet card, or assign lower one for something like
mouse.


    Current Xen do things similar.

Eddie

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