Interrupt forwarding

[Jon Smirl <jonsmirl@gmail.com>]

The subject of interrupt forwarding just came up on lkml. How is this
dealt with in Xen? I'm not currently a Xen user so I may be
misunderstanding how it works. In Xen I believe you can assign a piece
of hardware for exclusive use from a domain. How does this work for
shared interrupts, what if the domain receiving the interrupt dies and
can't acknowledge it?

Here's my comment from lkml....

On Sat, 12 Mar 2005 10:55:21 -0500, Jon Smirl <jonsmirl@gmail.com> wrote:
> I've tried implementing this before (on UML) and could not get around the
> interrupt problem. Most interrupts on the x86 architecture are shared.
> Disabling the IRQ at the PIC blocks all of the shared IRQs. This works
> (hope your userspace handler is last on the shared handler list) until
> you have a problem in userspace.
> 
> Once you have a problem in userspace there is no way to acknowledge
> the interrupt anymore. I tried to address that by maintaining a timer
> and suspending the hardware through the D0 state to reset it. That had
> some success. Not acknowledging the interrupt results in an interrupt
> loop and reboot.
> 
> The problem can be mitigated by choosing what slot your hardware to
> put your hardware in. This can reduce the number of shared interrupts.
> If you can get exclusive use of the interrupt this method will work.
> 
> If I were designing a new bus I would make interrupt acknowledge part
> of PCI config space in order to allow a single piece of code to
> acknowledge them. Since we can't change the bus the only safe way to
> do this is to build a hardware specific driver for each device to
> acknowledge the interrupt.
> 
> Bottom line is that I could find no reliable solution for handing interrupts.


-- 
Jon Smirl
jonsmirl@gmail.com


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