Re: PCI passthrough (shared IRQ) not working / contradicting information

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2013-05-03 09:33, Binarus wrote:
> Hi all,
> 
> a while ago, I have set up a KVM virtual machine with PCI passthrough.
> The device which is passed through is an AVM Fritz!card PCI v2.1.
> 
> This works, but there is a drawback: At that time, KVM was not able to
> do PCI passthrough for PCI devices with shared interrupt. So I had to
> find out which other devices were using the same interrupt, and had to
> inactivate them by handling them over to the PCI stub driver.
> 
> Then, one day, I have been very excited when I was reading that with
> kernel 3.4, KVM should be able to do PCI passthrough even if the
> respective device shares its IRQ with other devices. Thus, two day ago,
> I took the time, got vanilla kernel 3.9, configured, compiled and
> installed, and got qemu 1.4.1, configured, compiled and installed.
> 
> Now, I am disappointed that the passthrough still doesn't work if I
> leave the other devices active, probably due to a silly error on my
> side. It still works if I inactivate the other devices which share their
> IRQ with the AVM card, but that actually was not the goal.
> 
> Furthermore, the last two sentences on the following page suggest that
> PCI passthrough of devices with shared IRQs is still not possible:
> http://www.linux-kvm.org/page/How_to_assign_devices_with_VT-d_in_KVM. So
> there is a contradiction between what has been stated when kernel 3.4
> came out and what linux-kvm.org says.
> 
> Could anybody please tell me if PCI passthrough with shared IRQs is
> possible or not? Does anybody know why the AVM Fritz!card won't work
> without inactivating other devices? What mistake could lead to the
> current situation (passthrough works if other devices are inactivated,
> and does not work if not)?

The Fritz!CARD is very old and likely not PCI 2.3 compliant. But that is
required to share legacy interrupts.

Alex, does VFIO provide any convenient diagnostics if INTx sharing is
possible or not? If not, maybe we should add it at least over there.

Jan

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