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Re: [Qemu-discuss] [Qemu-devel] pci-assign fails with read error on conf

From: Daniel P. Berrange
Subject: Re: [Qemu-discuss] [Qemu-devel] pci-assign fails with read error on config-space file
Date: Wed, 2 Nov 2016 09:54:16 +0000
User-agent: Mutt/1.7.1 (2016-10-04)

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

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