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device_del command is not synchronous for PCI devices, it merely asks the guest to release the device and returns. If the host wants to use that device before the guest actually releases it, we are in big trouble. To avoid this, we already added a loop which waits up to 10 seconds until the device is actually released before we do anything else with that device. But we only added this loop for managed PCI devices before we try reattach them back to the host. However, we need to wait even for non-managed devices. We don't reattach them automatically, but we still want to prevent the host from using it. This was revealed thanks to sVirt: when we relabel sysfs files corresponding to the PCI device before the guest finished releasing the device, qemu is no longer allowed to access those files and if it wants (as a result of guest's request) to write anything to them, it just exits, which kills the guest. This is not a proper fix and needs some further work both on libvirt and qemu side in the future.
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>