[libvirt] hypervisor feature - hardware assisted paging

[Chris Lalancette]

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.

However, I would caution you to beware adding the hap flag unconditionally.
Especially on older Xen distributions (I'm thinking something like RHEL-5),
sending a hap value that xend does not understand may cause domain creation to
fail.

-- 
Chris Lalancette