[Xen-devel] Re: [PATCH][FIX] Possible fix for spurious interrupts

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 20 Apr 2006, at 08:45, Jan Beulich wrote:

> ... and even worse when the interrupt belongs to a domU.
>
> Have you heard anything from Intel on this? I am still hoping that 
> there is a pattern which interrupt gets triggered by
> masking a certain other one, so a (perhaps more complex, but less 
> impacting) fix could be derived...
Arun Sharma pointed out its the same problem that FreeBSD has been 
having (they also mask interrupt lines when the ISR is scheduled in an 
interrupt thread). The problem is caused *by design* on many chipsets 
-- the idea is that if the bootloader or OS does not know about ACPI 
and cannot find the IO-APICs then interrupt lines routed to those 
IO-APICs should have their interrupt fired on a legacy line (it so 
happens that this usually is also the line used by the USB 
controllers). They detect this by seeing if the IO-APIC RTE is masked. 
Many chipsets have no way to disable this mode of operation (no need 
for Linux and Windows -- when they detect and enable IO-APICs, they 
never mask active RTEs). So, basically, we're screwed and have to work 
around by delaying the EOI.
 -- Keir


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