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

On Fri, Oct 28, 2016 at 01:28:19PM +0200, 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...' > > I know i should probably not be doing this, 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. > > 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. errno == 22, means EINVAL, so it feels unlikely to be a permissions problem unless the kernel or QEMU is reporting the wrong errno. > 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. The 'config' file is a plain file, so not affected by cgroups - that only affects block devices. When libvirt runs QEMU, it runs unprivileged qemu:qemu user/group, so perhaps it is a permissions thing, despite the fact that you're getting EINVAL, not EACCESS.

It would be interesting to know just what part of the config space QEMU was trying to read I guess, to better understand why it might be failing