[libvirt-users] Question about disabling '3dnowprefetch' CPU feature in Xen Guest using libvirt

[Jim Fehlig]

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