[libvirt-users] [Qemu-devel] pci-assign fails with read error on config-space file

Alex Williamson alex.williamson at redhat.com
Fri Oct 28 17:08:19 UTC 2016

On Fri, 28 Oct 2016 11:25:55 -0400
Laine Stump <laine at redhat.com> wrote:

> On 10/28/2016 07:28 AM, Henning Schild wrote:
> > Hey,
> >
> > i am running an unusual setup where i assign pci devices behind the
> > back of libvirt. I have two options to do that:
> > 1. a wrapper script for qemu that takes care of suid-root and appends
> > arguments for pci-assign
> > 2. virsh qemu-monitor-command ... 'device_add pci-assign...'  
> With any reasonably modern version of Linux/qemu/libvirt, you should not 
> be using pci-assign, but should use vfio-pci instead. pci-assign is old, 
> unmaintained, and deprecated (and any other bad words you can think of).
> Also, have you done anything to lock the guest's memory in host RAM? 
> This is necessary so that the source/destination of DMA reads/writes is 
> always present. It is done automatically by libvirt as required *when 
> libvirt knows that a device is being assigned to the guest*, but if 
> you're going behind libvirt's back, you need to take care of that 
> yourself (or alternately, don't go behind libvirt's back, which is the 
> greatly preferred alternative!)

Note that pci-assign doesn't care about user locked memory limits, so
that much is not required for this deprecated use case, but I fully
agree that going behind libvirt's back is completely unadvised and
pci-assign is deprecated and likely broken.  Maybe we should even
consider removing it in the QEMU2.8 release cycle.  Use vfio-pci and
use the mechanisms provided in libvirt to attach the device to the VM,
if these don't work, file bugs and improve the environment to meet your
needs rather than working around it.  I can't figure out from the
original report what specifically about the environment prevents
use of libvirt <hostdev> entries.  Thanks,


> >
> > I know i should probably not be doing this,  
> Yes, that is a serious understatement :-) And I suspect that it isn't 
> necessary.
> >   it is a workaround to
> > introduce fine-grained pci-assignment in an openstack setup, where
> > vendor and device id are not enough to pick the right device for a vm.  
> libvirt selects the device according to its PCI address, not vendor and 
> device id. Is that not "fine-grained" enough? (And does OpenStack not 
> let you select devices based on their PCI address?)
> >
> > In both cases qemu will crash with the following output:
> >  
> >> qemu: hardware error: pci read failed, ret = 0 errno = 22  
> > followed by the usual machine state dump. With strace i found it to be
> > a failing read on the config space file of my device.
> > /sys/bus/pci/devices/0000:xx:xx.x/config
> > A few reads out of that file succeeded, as well as accesses on vendor
> > etc.
> >
> > Manually launching a qemu with the pci-assign works without a problem,
> > so i "blame" libvirt and the cgroup environment the qemu ends up in.
> > So i put a bash into the exact same cgroup setup - next to a running
> > qemu, expecting a dd or hexdump on the config-space file to fail. But
> > from that bash i can read the file without a problem.
> >
> > Has anyone seen that problem before?  
> No, because nobody else (that I've ever heard) is doing what you are 
> doing. You're going around behind the back of libvirt  (and OpenStack) 
> to do device assignment with a method that was replaced with something 
> newer/better/etc about 3 years ago, and in the process are likely 
> missing a lot of the details that would otherwise be automatically 
> handled by libvirt.
> > Right now i do not know what i
> > am missing, maybe qemu is hitting some limits configured for the
> > cgroups or whatever. I can not use pci-assign from libvirt, but if i
> > did would it configure cgroups in a different way or relax some limits?
> >
> > What would be a good next step to debug that? Right now i am looking at
> > kernel event traces, but the machine is pretty big and so is the trace.  
> My recommendation would be this:
> 1) look at OpenStack to see if it allows selecting the device to assign 
> by PCI address. If so, use that (it will just tell libvirt "assign this 
> device", and libvirt will automatically use VFIO for the device 
> assignment if it's available (which it will be))
> 2) if (1) is a deadend (i.e. OpenStack doesn't allow you to select based 
> on PCI address), use your "sneaky backdoor method" to do "virsh 
> attach-device somexmlfile.xml", where somexmlfile.xml has a proper 
> <hostdev> element to select and assign the host device you want. Again, 
> libvirt will automatically figure out if VFIO can be used, and will 
> properly setup everything necessary related to cgroups, locked memory, etc.
> >
> > That assignment used to work and i do not know how it broke, i have
> > tried combinations of several kernels, versions of libvirt and qemu.
> > (kernel 3.18 and 4.4, libvirt 1.3.2 and 2.0.0, and qemu 2.2.1 and 2.7)
> > All combinations show the same problem, even the ones that work on
> > other machines. So when it comes to software versions the problem could
> > well be caused by a software update of another component, that i
> > got with the package manager and did not compile myself. It is a debian
> > 8.6 with all recent updates installed. My guess would be that systemd
> > could have an influence on cgroups or limits causing such a problem.  
> That you would need to think of such things points out that your current 
> setup is fragile and ultimately unmaintainable. Please consider 
> "coloring inside the lines" :-) (We'd be happy to help if there are any 
> hangups along the way, either on the libvirt-users mailing list or in 
> the #virt channel on irc.oftc.net).

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