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Date:	Wed, 9 Jul 2014 15:05:56 +0100
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>
CC:	<konrad@...nel.org>, <boris.ostrovsky@...cle.com>,
	<linux-kernel@...r.kernel.org>, <xen-devel@...ts.xenproject.org>
Subject: Re: [Xen-devel] [PATCH v3 1/7] xen-pciback: Document the various
 parameters and attributes in SysFS

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@...ts.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.

~Andrew
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