[libvirt] hypervisor feature - hardware assisted paging

[Jim Fehlig]

Chris Lalancette wrote:
> On 01/04/11 - 10:41:07AM, Jim Fehlig wrote:
>   
>> I'm looking into a bug where a libvirt-created xen HVM guest boots
>> *very* slowly on an EPT-enabled machine, particularly when the guest has
>> a dedicated PCI device.
>>
>> xen-unstable c/s 16931 [1] introduced a per-guest HAP (hardware assisted
>> paging) setting.  Unfortunately, the sane default of hap=1 is done by xm
>> client tool instead of xend, so an HVM guest created with xm tool
>> (hap=1) boots as expected but takes much longer when created through
>> libvirt (hap=0).
>>
>> Should libvirt expose this feature in domain XML?  Should it be possible
>> to enable/disable hardware assisted paging on a per-guest level with
>> libvirt?  If so, where does such a setting belong?  It seems analogous
>> to enabling/disabling PAE and as such belongs under hypervisor
>> features.  On IRC, Eric suggested it might fall under memoryBacking like
>> huge page support in KVM.
>>
>> If folks feel this setting should not be exposed, it could be
>> unconditionally set in the xen driver for HVM guests, similar to the way
>> usb setting is currently handled (see attached patch).  Note that the
>> xen domain builder will ignore this setting on hardware without the feature.
>>     
>
> My personal feeling is that this is something we should expose.  It is useful
> for testing sometimes, so I think that we could expose it under the hypervisor
> features with apic, acpi, etc like you suggest.
>   

I sent a small patch set to the list which adds HAP to hypervisor features

https://www.redhat.com/archives/libvir-list/2011-January/msg00157.html

> However, I would caution you to beware adding the hap flag unconditionally.
>   

Right.  The patches just expose the setting - users must specify it. 
Could you review those? :)

Thanks!
Jim