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Date:	Thu, 22 May 2014 16:36:24 +0300
From:	Laine Stump <laine@...ne.org>
To:	Alex Williamson <alex.williamson@...hat.com>, bhelgaas@...gle.com,
	linux-pci@...r.kernel.org
CC:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	konrad.wilk@...cle.com, kim.phillips@...aro.org,
	gregkh@...uxfoundation.org, agraf@...e.de,
	stuart.yoder@...escale.com, libvir-list@...hat.com,
	iommu@...ts.linux-foundation.org, tech@...tualopensystems.com,
	kvmarm@...ts.cs.columbia.edu, christoffer.dall@...aro.org
Subject: Re: [libvirt] [PATCH v3] PCI: Introduce new device binding path using
 pci_dev.driver_override

On 05/20/2014 05:53 PM, Alex Williamson wrote:
> The driver_override field allows us to specify the driver for a device
> rather than relying on the driver to provide a positive match of the
> device.  This shortcuts the existing process of looking up the vendor
> and device ID, adding them to the driver new_id, binding the device,
> then removing the ID, but it also provides a couple advantages.
>
> First, the above existing process allows the driver to bind to any
> device matching the new_id for the window where it's enabled.  This is
> often not desired, such as the case of trying to bind a single device
> to a meta driver like pci-stub or vfio-pci.  Using driver_override we
> can do this deterministically using:
>
> echo pci-stub > /sys/bus/pci/devices/0000:03:00.0/driver_override
> echo 0000:03:00.0 > /sys/bus/pci/devices/0000:03:00.0/driver/unbind
> echo 0000:03:00.0 > /sys/bus/pci/drivers_probe

I'm not qualified to review the mechanics of the patch, but as a
potential consumer of this new interface (in libvirt), I want to say
that it is *immensely* improved over the racy, error prone method that
the kernel currently has available. The first day that I looked at the
libvirt code that uses the "new_id" method to bind drivers to devices, I
wished that there would instead be an interface more or less identical
to what Alex has described here - simple, deterministic, and with
minimal side effect outside of the exact device and driver pair we want
to bind.

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