[Xen-devel] Interrupts in Xen and GuestOS

[Rami Rosen <rosenrami@xxxxxxxxx>]

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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