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Re: [libvirt-users] PCI-Passthrough suddenly stopped working



On 07/11/2012 04:24 PM, Rouven Sacha wrote:
> Hi everybody!
> 
> On a Centos 5.8 with libvirt qemu-0.10.5-1.el5.2, libvirt-0.8.2-22.el5
> and 2.6.18-308.11.1.el5 I have a Windows machine that used to have
> pci-passthrough for a device enabled and working.
> 
> After an update (kernel, libvirt afaik) and  a reboot, pci-passthrough
> stopped working. Intel IOMMU is enabled (can see that from the dmesg
> output).

This may be the result of a security fix in the new kernel.  I know at
least one older version of Intel chips has a bug where IOMMU can be
exploited by a guest to take control over the host, so on those chips,
newer kernels now require to explicitly enable a kernel module parameter
to state that you are going to allow passthrough to the guest in spite
of the security risk.  That is, you may need to use:

modprobe kvm allow_unsafe_assigned_interrupts=1

with your newer kernel.  Unfortunately, I wasn't able to find a better
URL to a page documenting this issue, so that implies we probably also
need a patch to the libvirt documentation with regards to using device
passthrough.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org



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