Hi, On Thu, May 12, 2011 at 15:39:44 -0700, Dong-In David Kang wrote:

I'm interested in getting native capabilities of host CPU using libvirt. I've looked at .../src/cpu/cpu_map.xml. And it does describe native capabilities of host CPU very well. But, libvirt always returns a subset of the native capabilities which is far different from the feature lists in the xml file. (I'm testing with <model name='Westmere'>.) The sponsor of my project is interested is high-performance computing. And knowing native feature of the host CPU that can also be available to a virtual machine is very important for performance reason.

I couldn't find how to get the native features of the host that is described in .../src/cpu/cpu_map.xml. Is there a way or libvirt API to get the native capabilities of host CPU that is well described in .../src/cpu/cpu_map.xml?

I'm not sure I totally understand what you are looking for. Is the issue you have that virsh capabilities tells you something like this: <cpu> <arch>x86_64</arch> <model>Westmere</model> <vendor>Intel</vendor> <topology sockets='1' cores='2' threads='2'/> <feature name='rdtscp'/> <feature name='xtpr'/> <feature name='tm2'/> <feature name='est'/> <feature name='vmx'/> <feature name='ds_cpl'/> <feature name='monitor'/> <feature name='pbe'/> <feature name='tm'/> <feature name='ht'/> <feature name='ss'/> <feature name='acpi'/> <feature name='ds'/> <feature name='vme'/> </cpu> while in cpu_map.xml mentions much more features for Westmere model? If so, than the answer is quite simple. The thing is that the CPU description is additive, that is the above XML snippet says that host CPU supports all features that are defined by Westmere CPU model in cpu_map.xml *plus* the features that are explicitly listed in the above <cpu> element. So if you expand the model, you get a full list of features supported by the host CPU: <!-- from Nehalem which Westmere is derived from --> <feature name='sse2'/> <feature name='sse'/> <feature name='fxsr'/> <feature name='mmx'/> <feature name='pat'/> <feature name='cmov'/> <feature name='pge'/> <feature name='sep'/> <feature name='apic'/> <feature name='cx8'/> <feature name='mce'/> <feature name='pae'/> <feature name='msr'/> <feature name='tsc'/> <feature name='pse'/> <feature name='de'/> <feature name='fpu'/> <feature name='mtrr'/> <feature name='clflush'/> <feature name='mca'/> <feature name='pse36'/> <feature name='pni'/> <feature name='cx16'/> <feature name='ssse3'/> <feature name='sse4.1'/> <feature name='sse4.2'/> <feature name='popcnt'/> <feature name='lm'/> <feature name='syscall'/> <feature name='nx'/> <feature name='lahf_lm'/> <!-- from Westmere --> <feature name='aes'/> <!-- explicitly listed --> <feature name='rdtscp'/> <feature name='xtpr'/> <feature name='tm2'/> <feature name='est'/> <feature name='vmx'/> <feature name='ds_cpl'/> <feature name='monitor'/> <feature name='pbe'/> <feature name='tm'/> <feature name='ht'/> <feature name='ss'/> <feature name='acpi'/> <feature name='ds'/> <feature name='vme'/> Jirka

On Thu, May 12, 2011 at 03:39:44PM -0700, Dong-In David Kang wrote:

Hello,

I'm interested in getting native capabilities of host CPU using libvirt. I've looked at .../src/cpu/cpu_map.xml. And it does describe native capabilities of host CPU very well. But, libvirt always returns a subset of the native capabilities which is far different from the feature lists in the xml file. (I'm testing with <model name='Westmere'>.) The sponsor of my project is interested is high-performance computing. And knowing native feature of the host CPU that can also be available to a virtual machine is very important for performance reason.

I couldn't find how to get the native features of the host that is described in .../src/cpu/cpu_map.xml.

Libvirt provides a set of APIs for CPU compatibility comparisons, such that you don't actually need to know the precise CPU flags. The 'virsh capabilities' command returns an XML description of the host. This description takes the form of a CPU model that has the closest feature set to your physical CPU, plus a set of flags to make up the remainining delta. You can put that host CPU XML description unchanged in the guest XML and the guest will use the same set of features as the host. If you have a different CPU XML description for the guest CPU, you can also query whether it is supported by the host, eg cat > cpu.xml <cpu> <model>Penryn</model> <feature name='xtpr'/> <feature name='tm2'/> <feature name='est'/> <feature name='vmx'/> <feature name='ds_cpl'/> <feature name='monitor'/> <feature name='pbe'/> </cpu> <<EOF $ virsh cpu-compare cpu.xml CPU described in cpu.xml is incompatible with host CPU There is much more you can do besides this, see this guide http://berrange.com/posts/2010/02/15/guest-cpu-model-configuration-in-libvir... Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|