Xen devel - Sep 2005 - Interrupts in Xen and GuestOS

Hello,

Something with interrupts in Xen and GuestOS in unclear to me:

Let''s take for example a case where a frame arrives and we have
interrupt 11
on the 
physical device , the ETH NIC.

we get to do_irq() in xen/arch/x86/irq.c 
Now there ,after acknowledging the irq controller (the 8259 APIC) ,
as also is done in a NON-XEN kernel,it performs a check:

if ( likely(desc->status & IRQ_GUEST) )
and delegates to guest OS if the condition is true.

desc is an instance of irq_desc_t. 

In most of cases, as the unlikely hints,the IRQ_GUEST is set.
What I would like to know is , in which case the interrupt will reach
do_irq() and the IRQ_GUEST **WILL NOT** be set. 

It seems to me that all NIC interrutps should reach do_irq when desc->status 
& IRQ_GUEST 
is TRUE. Am I right ? 
Can it happen that ordinary interrupts of the NIC (interrupt 11 in this 
example)
will reach do_IRQ() without this flag set ? 

Regards,
Rami


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>   It seems to me that all NIC interrutps should reach do_irq when 
> desc->status & IRQ_GUEST
>  is TRUE. Am I right ?  
>   Can it happen that ordinary interrupts of the NIC (interrupt 11 in 
> this example)
>   will reach do_IRQ() without this flag set ?
No. Xen only handles timer and serial interrupts. All other I/O devices 
are handled by guests.

  -- Keir


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