From: Alex Williamson <alex.williamson@redhat.com>
Subject: Re: Device assignment, shared IRQs, uio_pci_generic
Date: Sat, 30 Oct 2010 11:06:04 -0600
Message-ID: <1288458364.2893.16.camel@x201>
References: <4CCC40A6.7080008@web.de>
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Cc: "Michael S. Tsirkin" <mst@redhat.com>, kvm <kvm@vger.kernel.org>
To: Jan Kiszka <jan.kiszka@web.de>
In-Reply-To: <4CCC40A6.7080008@web.de>
Sender: kvm-owner@vger.kernel.org

On Sat, 2010-10-30 at 17:58 +0200, Jan Kiszka wrote:
> Hi all,
> 
> trying to pass some host device to a KVM guest (for driver hacking), I
> ran into massive IRQ sharing issues on the host. Classic device
> assignment refuses to work in this case. I remembered uio_pci_generic,
> which I thought was once designed to resolve this conflict. But it isn't
> used by KVM or qemu, is it?

Nope, VFIO started out being based on it, but has since branched off.
KVM has never made use of it.

> So I crossed fingers and hacked IRQF_SHARED into
> assigned_device_enable_host_intx. Effect as expected: assignment work,
> host gets massive hick-ups (the graphic adapter is another user of this
> line...).
> 
> Are there already VFIO patches ready for testing that work with shared
> IRQs as well? And/or would it make sense to integrate IRQ sharing via
> PCI-level IRQ masking to the KVM core until VFIO becomes standard?

There's a very old qemu VFIO patch floating around, but I plan to send
out a new one this week, thus all the pre-patches I've been sending out.
Adding PCI 2.3 interrupt disable support to KVM should be very possible,
and I suspect fairly easy.  Oddly, I've just been adding support for
non-PCI 2.3 devices to VFIO so it can support all the old devices that
KVM does already.

Alex