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Date: Wed, 9 Jul 2014 15:57:15 +0100 From: David Vrabel <david.vrabel@...rix.com> To: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> CC: Andrew Cooper <andrew.cooper3@...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 15:47, 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@...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. >>> >>> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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