Chapter 16. Virtualization

Support has been added for qemu-kvm trace events during the virtual machine system shutdown process, which allows users to get detailed diagnostics about a guest system's shutdown requests issued by the virsh shutdown command or by the virt-manager application. This provides users with enhanced capabilities for isolating and debugging KVM guest problems during shutdown.

With this update, qemu-kvm allows the Intel Memory Protection Extensions (MPX) feature to be exposed to the guest. On the Intel 64 host systems that support MPX, this enables the use of a set of extensions that provide hardware support for bounds protection on pointer references.

The dump-guest-memory.py script has been introduced into QEMU, which makes it possible to analyze a guest memory dump from the qemu-kvm core in case of a guest kernel failure. For further information, see the related help text by using the help dump-guest-memory command.

With Red Hat Enterprise Linux 7.2, the virt-v2v command-line tool has become fully supported. This tool converts virtual machines running on foreign hypervisors to run on KVM. Currently, virt-v2v can convert Red Hat Enterprise Linux and Windows guests running on Red Hat Enterprise Linux 5 Xen and VMware vCenter.

Red Hat Enterprise Linux with KVM is supported on AMD64 and Intel 64 systems, but not on IBM Power Systems. Red Hat currently provides a POWER8-based solution with Red Hat Enterprise Virtualization for IBM Power Systems.

More information on version support and installation procedures can be found in the following Knowledge Base article: https://access.redhat.com/articles/1247773.

Now it is possible to use Thin Provisioned Hyper-V virtual hard disk (VHDX). The update adds support to shrink the underlining VHDX files for Microsoft Hyper-V virtual machines to actual used size.

KVM can now use the tcmalloc library, which provides a significant performance improvement in I/O operations per second.

When live migrating a domain as well as its disks, the user can now select which disks are copied during the migration. This allows for more efficient live migration when copying certain disks is undesirable, such as when they already exist on the destination, or when they are no longer useful.

Under the changes made to Red Hat Enterprise Linux 7.1, when an attempt is made to assign a device that is entangled by an Reserved Memory Region Reporting (RMRR) association, the kernel reports the following error in the dmesg log:

"Device is ineligible for IOMMU domain attach due to platform RMRR requirement. Contact your platform vendor."

The platform vendor has the ability to request that the VT-d IOMMU subsystem within the kernel retain specific mappings for devices using entries in the Advanced Configuration and Power Interface Direct Memory Access Remapping (ACPI DMAR) table known as RMRR structures. However, QEMU-KVM and VFIO have no visibility to these mapping requirements and no API exists to disable any potential ongoing communication that may occur through these regions. Therefore, a device associated with an RMRR could continue to use DMA through this address space even after the device was assigned to a guest VM. This could cause a device to overwrite VM memory with DMA data intended for the memory described by the RMRR.

To fix this bug, devices with associated RMRRs are excluded from participating in the kernel internal IOMMU API. Users can now identify such devices using dmesg logs and are also protected from assignment of devices making use of mappings which have the ability to cause instability within guest virtual machines. Users prevented from making use of PCI device assignment as a result of this change should contact their platform vendor for a BIOS update to release the I/O device from the imposed RMRR requirement.

For further information on these changes, see the following Knowledgebase article:

https://access.redhat.com/articles/1434873

The Microsoft Azure Linux Agent (WALA) version 2.0.13 has been included in the Extras channel. This agent supports the provisioning and running of Linux Virtual Machines in the Windows Azure cloud and should be installed on Linux images that are built to run in the Windows Azure environment.