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 :| -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
