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

On Wed, Jul 09, 2014 at 03:22:30PM +0100, Andrew Cooper wrote:
> On 09/07/14 15:13, Konrad Rzeszutek Wilk wrote:
> > 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@xxxxxxxxxxxxxxxxxxxx
> >>>>> +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.
> Xen will (and by this I mean 'should', and this was the behaviour last
> time I delved in there) only mask the IRQ if dom0 is the only consumer
> of these interrupts.
> For any PCIPassthrough, dom0 will get line interrupts for passed-through
> devices, but in this case pci-back should always handle the line
> interrupts so Linux doesn't block them as an IRQ storm.

And that is what it does - and this option provides the option to enable/disable
it the system admin wishes to do it.
> ~Andrew

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