6.3. x86-64 Architectures

Enhancements/Updates

The hypervisor can now support up to 512GB of memory. To ensure that the system properly detects and uses this much memory, boot the virtualized kernel with the parameter xenheap_megabytes=64.
For systems that use Non-Uniform Memory Access (NUMA), Red Hat recommends that you use the parameter dom0_mem=512m. Note that this can be set to a maximum of 120GB (dom0_mem=120gb).
You can now run paravirtualized 32-bit guests on 64-bit hosts. This capability is now included as a technology preview. Note that the ability to save, restore, and migrate paravirtualized 32-bit guests on 64-bit hosts is not functional, and as such should not be attempted.

Known Issues

Migrating paravirtualized guests through xm migrate [domain] [dom0 IP address] does not work.
Creating a guest more than 1,000 times will cause dom0 to reboot suddenly. Red Hat recommends that you pre-empt this by rebooting dom0 before the domain ID of any guest reaches 1000.
Installing the Virtualization feature may cause a
```
time went backwards
```
warning on HP systems with model numbers xw9300 and xw9400.
To work around this issue for xw9400 machines, configure the BIOS settings to enable the HPET timer. Note that this option is not available on xw9300 machines.
Installing Red Hat Enterprise Linux 3.9 on a fully virtualized guest may be extremely slow. In addition, booting up the guest after installation may result in hda: lost interrupt errors.
To avoid this bootup error, configure the guest to use the SMP kernel.
Upgrading a host (dom0) system to Red Hat Enterprise Linux 5.2 may render existing Red Hat Enterprise Linux 4.5 SMP paravirtualized guests unbootable. This is more likely to occur when the host system has more than 4GB of RAM.
To work around this, boot each Red Hat Enterprise Linux 4.5 guest in single CPU mode and upgrade its kernel to the latest version (for Red Hat Enterprise Linux 4.5.z).