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

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Message-ID: <53BD5026.2030908@citrix.com>
Date:	Wed, 9 Jul 2014 15:22:30 +0100
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	David Vrabel <david.vrabel@...rix.com>, <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 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@...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.
> 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.

~Andrew
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