On Tue, Dec 19, 2017 at 08:44:48PM +0100, Marek Marczykowski-Górecki wrote:
On Tue, Dec 19, 2017 at 01:45:57PM +0000, Daniel P. Berrange wrote:
On Tue, Dec 19, 2017 at 01:43:24PM +0000, Joao Martins wrote:
On 12/19/2017 01:13 PM, Daniel P. Berrange wrote:
On Tue, Dec 19, 2017 at 01:01:36PM +0000, Joao Martins wrote:
[Sorry for double posting, but I mistakenly forgot to include libvirt list)
+WimT +Daniel
On 12/10/2017 02:10 AM, Marek Marczykowski-Górecki wrote:
<cpu mode='host-passthrough'> element may be used to configure other features, like NUMA, or CPUID. Do not enable nested HVM (which is in "preview" state after all) by mere presence of <cpu mode='host-passthrough'> element, but require explicit <feature policy='force' name='vmx'/> (or 'svm'). Also, adjust xenconfig driver to appropriately translate to/from nestedhvm=1.
While at it, adjust xenconfig driver to not override def->cpu if already set elsewhere. This will help with adding cpuid support.
I agree with this and it was what we came up in the first version of nested hvm support[0]. Although Daniel suggested there to use the same semantics of qemu driver such that host-passthrough enables nested hvm without the use of:
<feature policy='require' name='vmx'/>
Yes, the key point of libvirt is to apply consistent semantics across different drivers, so we should not diverge betweeen QEMU & Xen in this regard.
/nods
'host-passthrough' and 'host-model' are supposed to expose *every* feature that the host CPUs support (except for those few which the hypervisor may block due to ability to virtualize them).
So 'host-passthrough' is correct to automatically expose vmx/svm, without requiring any extra <feature> element, and I don't think we can accept this patch.
My point is you can use <cpu> element to configure various features, like mentioned above (NUMA etc). As discussed previously, in libxl driver only 'host-passthrough' mode makes sense, because that's what libxl allows (enabled/disable various features, not define the whole CPU). So, you can use something like:
<cpu mode='host-passthrough'> <numa> <cell id='0' cpus='0-3' memory='512000' unit='KiB'/> <cell id='1' cpus='4-7' memory='512000' unit='KiB' memAccess='shared'/> </numa> </cpu>
Now, this is _very not obvious_ you've just enabled potentially dangerous feature. Quoting https://wiki.xenproject.org/wiki/Nested_Virtualization_in_Xen:
This means an L1 admin can DOS the L0 hypervisor. This is a potential security issue; for this reason, we do not recommend running nested virtualization in production yet.
Enabling potentially harmful features without explicit consent is not something that I'd expect from a project meant to be used in production environment...
Whoever wrote that XML *has* given explicit consent, because that is applying the documented semantics of the 'host-passthrough' CPU mode. This is exactly the same situation as with the KVM driver. The mistake here is assuming that mode='host-passthrough' is identical to not listing CPUJ at all. If you don't want VMX/SVM added when you define NUMA, then do <cpu mode='host-passthrough'> <feature name="vmx" policy="disable"/> <numa> <cell id='0' cpus='0-3' memory='512000' unit='KiB'/> <cell id='1' cpus='4-7' memory='512000' unit='KiB' memAccess='shared'/> </numa> </cpu> In retrospect the <numa> info should not have been inside the <cpu> element, but that's something we unfortunately have to live with now for back compatibility.
Generally I think this is bad idea that placing just <cpu mode='host-passthrough'>, without any specific setting, change anything (compared to no <cpu/> at all). At least in context of libxl driver.
There's nothing specific about libxl there - it would do the same for KVM too if the host supports svm/vmx.
You could conceivably replicate the host-level control KVM has by using an /etc/libvirt/libxl.conf driver level config option to indicate whether nested-virt is permitted or not.
That could work. Is 'nestedhvm' ok for parameter name (disabled by default)?
Sure, whatever parameter name you feel is best - there's no rules about parameters / naming for the driver specific global config files. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|