Charles Shih (史晨)
Quality Engineer
Red Hat, Platform QE, Virt QE, Section 1
Email: cheshi@redhat.com
IRC: cheshi @ #eng-china, #hyperv ,#qa, #virt
T: +86 10 65627484  - IP: 8387484
M: +86 18611268098

On Wed, Jul 5, 2017 at 11:42 PM, Jim Fehlig <jfehlig@suse.com> wrote:

On 06/18/2017 09:01 PM, Charles Shih wrote:

Dear All,

I'm reaching this mail-list to ask a small question about disabling '3dnowprefetch' CPU feature in Xen Guest using libvirt.

This is my environment:
Fedora release 26 (Twenty Six)
4.11.0-0.rc3.git0.2.fc26.x86_64
xen-4.8.1-2.fc26.x86_64
libvirt-3.2.1-1.fc26.x86_64

I can disable '3dnowprefetch' CPU feature in guest via 'xl' command, with adding `cpuid='host,3dnowprefetch=0'` into the CFG file.

However, follow the instruction (https://libvirt.org/formatdomain.html#elementsCPU <https://libvirt.org/formatdomain.html#elementsCPU>), I added the following block into my XML file:

```
<cpu mode='host-passthrough' check='none'>
<feature policy='disable' name='3dnowprefetch'/>
</cpu>
```

Created the instance, seemed this feature was not being disabled in my guest. (I was able to see '3dnowprefetch' in `lscpu` outputs)

Anybody has idea on this? Does libvirt support this for Xen?

Currently, the libvirt libxl driver only supports VMX or SVM CPU feature, and for that you'd need libvirt 3.3.0

http://libvirt.org/git/?p=libvirt.git;a=commit;h=c268b9eaeb4321ef311ae2e152b80752918eb289

More support for CPU customization is on the todo list. Contributions are always warmly welcomed :-).

Regards,
Jim