This email to the SPDK list is a follow-on to a brief discussion held during a recent SPDK community meeting (Tue Jun 26 UTC 15:00).

Lifted and edited from the Trello agenda item (https://trello.com/c/U291IBYx/91-best-practices-on-driver-binding-for-spdk-in-production-environments):

During development many (most?) people rely on the run of SPDK's scripts/setup.sh to perform a number of initializations, among them the unbinding of the Linux kernel nvme driver from NVMe controllers targeted for use by the SPDK and then binding them to either uio_pci_generic or vfio-pci.   This script is applicable for development environments, but not targeted for use in productions systems employing the SPDK.

I'd like to confer with my fellow SPDK community members on ideas, suggestions and best practices for handling this driver unbinding/binding.   I wrote some udev rules along with updates to some other Linux system conf files for automatically loading either the uio_pci_generic or vfio-pci modules.  I also had to update my initramfs so that when the system comes all the way up, the desired NVMe controllers are already bound to the needed driver for SPDK operation.  And, as a bonus, it should "just work" when a hotplug occurs as well.   However, there may be additional considerations I might have overlooked on which I'd appreciate input.   Further, there's the matter of how and whether to semi-automate this configuration via some kind of script and how that might vary according to Linux distro to say nothing of the determination of employing uio_pci_generic vs vfio-pci.

And, now some details:

1.  I performed this on an Oracle Linux (OL) distro.   I’m currently unaware how and what configuration files might be different depending on the distro.   Oracle Linux is RedHat-compatible, so I’m confident my implementation should run similarly on RedHat-based systems, but I’ve yet to delve into other distro’s like Debian, SuSE, etc.

2.  In preparation to writing my own udev rules, I unbound a specific NVMe controller from the Linux nvme driver by hand.   Then, in another window I launched:  "udevadm monitor -k -p” so that I could observe the usual udev events when a NVMe controller is bound to the nvme driver.   On my system, I observed four (4) udev kernel events (abbreviated/edited output to avoid this become excessively long):

(Event 1)
KERNEL[382128.187273] add      /devices/pci0000:00/0000:00:02.2/0000:30:00.0/nvme/nvme0 (nvme)
ACTION=add
DEVNAME=/dev/nvme0
SUBSYSTEM=nvme

(Event 2)
KERNEL[382128.244658] bind     /devices/pci0000:00/0000:00:02.2/0000:30:00.0 (pci)
ACTION=bind
DEVPATH=/devices/pci0000:00/0000:00:02.2/0000:30:00.0
DRIVER=nvme
SUBSYSTEM=pci

(Event 3)
KERNEL[382130.697832] add      /devices/virtual/bdi/259:0 (bdi)
ACTION=add
DEVPATH=/devices/virtual/bdi/259:0

...
SUBSYSTEM=bdi

(Event 4)
KERNEL[382130.698192] add      /devices/pci0000:00/0000:00:02.2/0000:30:00.0/nvme/nvme0/nvme0n1 (block)
ACTION=add
DEVNAME=/dev/nvme0n1
DEVPATH=/devices/pci0000:00/0000:00:02.2/0000:30:00.0/nvme/nvme0/nvme0n1
DEVTYPE=disk
...
SUBSYSTEM=block


3.   My udev rule triggers on (Event 2) above:   the bind action.   Upon this action, my udev rule appends operations to the special udev RUN variable such that udev will essentially mirror that which is done in the SPDK’s scripts/setup.sh for unbinding from the nvme driver and binding to, in my case, the vfio-pci driver.

4.   With my new udev rules in place, I was successful getting specific NVMe controllers (based on bus-device-function) to unbind from the Linux nvme driver and bind to vfio-pci.   However, I made a couple of observations in the kernel log (dmesg).   In particular, I was drawn to the following for an NVMe controller at BDF:  0000:40:00.0 for which I had a udev rule to unbind from nvme and bind to vfio-pci:

[   35.534279] nvme nvme1: pci function 0000:40:00.0
[   37.964945] nvme nvme1: failed to mark controller live
[   37.964947] nvme nvme1: Removing after probe failure status: 0

One theory I have for the above is that my udev RUN rule was invoked while the nvme driver’s probe() was still running on this controller, and perhaps the unbind request came in before the probe() completed hence this “name1: failed to mark controller live”.   This has left lingering in my mind that maybe instead of triggering on (Event 2) when the bind occurs, that perhaps I should instead try to derive a trigger on the “last" udev event, an “add”, where the NVMe namespace’s are instantiated.   Of course, I’d need to know ahead of time just how many namespaces exist on that controller if I were to do that so I’d trigger on the last one.   I’m wondering if that may help to avoid what looks like a complaint during the middle of probe() of that particular controller.   Then, again, maybe I can just safely ignore that and not worry about it at all?    Thoughts?

I discovered another issue during this experimentation that is somewhat tangential to this task, but I’ll write a separate email on that topic.

thanks for any feedback,

--
Lance Hartmann
lance.hartmann@oracle.com