[libvirt] [PATCH v1 0/3] libxl: nestedhvm support

[Joao Martins]

On 03/08/2017 02:13 PM, Daniel P. Berrange wrote:
> On Wed, Mar 08, 2017 at 03:07:12PM +0100, Wim Ten Have wrote:
>> From: Wim ten Have <wim.ten.have at oracle.com>
>>
>> This patch enhances host-passthrough capability to advertise the
>> required vendor CPU virtualization feature which will be used to
>> enable 'nestedhvm' in the libxl driver.
>>
>> For Intel virtualization (VT-x)
>>     ..
>>   <cpu mode='host-passthrough'>
>>     <feature policy='require' name='vmx'/>
>>   </cpu>
>>
>> For AMD virtualization (AMD-V)
>>     ..
>>   <cpu mode='host-passthrough'>
>>     <feature policy='require' name='svm'/>
>>   </cpu>
> 
> If using host-passthrough or host-model, and the host's CPU includes
> vmx or svm, then I would expecte nested-HVM to be enabled, without
> needing any extra <feature> flag in the XML. That would match the
> semantics used with the QEMU driver.
> 
> The only time we would need to use <feature> is if using mode=custom
> along with a named CPU model which lacks vmx/svm flags.
Ah OK - I was kinda off unclear on that. So using host-passthrough should be
enough then. (while making sure libxl checks if host->cpu does have vmx or svm
in its features)

> BTW, I wonder how hard it would be to wire up the libxl driver to use
> the CPU model infrastructure in src/cpu ? It feels a little odd to use
> XML <cpu mode='host-passthrough'/> if we're not then making sure it
> actually uses host-passthrough when configuring the guest.
While xen libxl do allow to mangle the cpuid, it is meant for disabling features
at this point. libxl follows a "host-model" first, meaning the default is to
expose as much as features as possible to the guest (depending on whether it's
PV or HVM). However, nested virt is a slightly special case since the toolstack
will do more than simply unmasking vmx/svm bits (actually within libxl, a sysctl
to Xen is performed to enable nested virt to the domain, in which case libxc
will handle any vendor specific bits). IOW, even when we improve libxl cpuid
policy management to the point we can wire up to libvirt cpu model
infrastruture, we would still need to handle the nestedhvm special case (which
btw this field would work even for ARM whenever supported).

Joao