Re: [Xen-devel] [PATCH v3 1/7] xen-pciback: Document the various parameters and attributes in SysFS

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Wed, Jul 09, 2014 at 03:05:56PM +0100, Andrew Cooper wrote:
> On 09/07/14 14:59, Konrad Rzeszutek Wilk wrote:
> >
> >>> +What:           /sys/bus/pci/drivers/pciback/irq_handler_state
> >>> +Date:           Oct 2011
> >>> +KernelVersion:  3.1
> >>> +Contact:        xen-devel@lists.xenproject.org
> >>> +Description:
> >>> +                An option to toggle Xen PCI back to acknowledge (or stop)
> >>> +                interrupts for the specific device regardless of whether the
> >>> +                device is shared, enabled, or on a level interrupt line.
> >>> +                Writing a string of DDDD:BB:DD.F will toggle the state.
> >>> +                This is Domain:Bus:Device.Function where domain is optional.
> >> I do not understand under what circumstances this should be used in.
> > So that dom0 does not disable the IRQ line as it would be getting the IRQs
> > for the guest as well (because the IRQ line is level and another guest
> > uses an PCI device that is using the same line).
> 
> Why is this relevant?  Xen (and Xen alone) actually controls this aspect
> of interrupts.  Xen manages passing line level interrupts to any domain
> which might have a device hanging off a particular line, and has to wait
> until all domains have EOI'd the line until it can clear the interrupt
> at the IO-APIC.

Because Linux will think there is an IRQ storm as the event->IRQ points
to the default one. And then it will mask the event, which means dom0
will mask the PIRQ, and Xen will then also mask the IRQ.
> 
> ~Andrew