Re: [libvirt-users] [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
Regards,
Daniel
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