RE: [Xen-devel] IRQ range in domain 0

["Petersson, Mats" <mats.petersson@xxxxxxx>]

1. The APIC that is included in almost every CPU produced today
(certainly all PC processors produced today) is capable of more than 15
inputs (as I understand it, there's 256 "inputs".

2. Interupts with numbers higher than 256 are "virtual interrupts",
which is a "XEN thing", and they are not REAL interrupts from hardware,
but signalling within XEN to (for example) wake up another Domain. 

This is to the best of my understanding, and I have spent no time trying
to trace, understand or research any of this, so if you're going to use
any of the above for anything serious, I suggest you do some reasearch
to confirm or revise the above statements.

--
Mats
> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of John Que
> Sent: 25 August 2005 12:13
> To: Xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-devel] IRQ range in domain 0
> 
> Hello,
>   I am running Xen-Unstable on intel x386 , with one CPU ; 
> When running cat /proc/interrupts on Domain 0  I see the following:
> 
>           CPU0
>   1:       1424        Phys-irq  i8042
>   9:          0        Phys-irq  acpi
>  14:     421055        Phys-irq  ide0
>  15:         16        Phys-irq  ide1
>  17:       4419        Phys-irq  eth0
>  20:      10884        Phys-irq  ohci_hcd:usb1
>  21:          0        Phys-irq  ohci_hcd:usb2
> 256:          0     Dynamic-irq  ctrl-if
> 257:      77411     Dynamic-irq  timer0
> 258:          0     Dynamic-irq  console
> 259:          0     Dynamic-irq  net-be-dbg
> NMI:          0
> LOC:          0
> ERR:          0
> MIS:          0
> 
> can I deduce from this that the number of available physical 
> IRQ is higher than 15?
> What is strange to me is that on an ordinary linux kernel 
> (not a xen-patched kernel ) , there can be up to 15 
> interrupts,and in all cases I know, they are in the range 0-14.
> I don't remeber I saw a phiyscal interrupt whose number is 
> higher than 14 on a standard x386 platform.
> 
> Also according to "Unerstanding the Linux Kernel" book,chapter 4: 
> "Tradional PICs are implemented by connecting "in cascade" 
> two 8259A-style external chips.Each chip can handle up to 
> eight different IRQ input lines. Since the INT output line of 
> the slave PIC is connected to the IRQ2 pin of the master PIC, 
> the number of available IRQ lines is limited to 15."
> 
> 
> Any ideas?
> Regards
> John Que.
> 
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> 


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