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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Jul 09, 2014 at 03:45:03PM +0100, David Vrabel wrote:
> On 09/07/14 15:25, Konrad Rzeszutek Wilk wrote:
> > 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.
> 
> I still don't understand why someone would want to flip the handler to
> a broken mode.

The intent was to allow you to flip to the 'enable' mode in case Linux
did not detect it correctly.
> 
> The original commit isn't very enlightening either.

Thoughts then on what this documentation patch should say to make it
clear of its intent?

> 
> David

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