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

[David Vrabel <david.vrabel@citrix.com>]

On 09/07/14 15:47, Konrad Rzeszutek Wilk wrote:
> 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@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.
>>>>
>>>> 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.

We should not provide sysfs knobs to work around kernel bugs.

>> The original commit isn't very enlightening either.
> 
> Thoughts then on what this documentation patch should say to make it
> clear of its intent?

I think it should be removed.

It also has non-standard behaviour of /toggling/ with every write
instead of using a write of a 1 or a 0 like every other sysfs file.

David