RE: [Xen-devel] Interrupt forwarding

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> 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?

The paper on IO on the Architecture section of the web site is probably
reference (other than the code). 

Shared interrupts aren't good news for performance, but they work OK.

If a driver domain locks up we can kill and restart it -- for safety we
don't rely on it to ack the interrupt.

Ian
 
> Here's my comment from lkml....
> 
> On Sat, 12 Mar 2005 10:55:21 -0500, Jon Smirl 
> <jonsmirl@xxxxxxxxx> 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@xxxxxxxxx
> 
> 
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