Re: [PATCH v2 02/18] x86/reboot: Expose VMCS crash hooks if and only if KVM_INTEL is enabled

[Huang, Kai]

> 
> > But I am not sure whether we want to do this (having CONFIG_KVM_INTEL around the
> > relevant code).  In later patches, you mentioned the case of out-of-tree
> > hypervisor, for instance, below in the changelog of patch 04:
> > 
> > 	There's no need to attempt VMXOFF if KVM (or some other out-of-tree�
> > 	hypervisor) isn't loaded/active...
> > 
> > This means we want to do handle VMCLEAR+VMXOFF in case of out-of-tree hypervisor
> > too.  So, shouldn't the hooks always exist but not only available when KVM_INTEL
> > or KVM_AMD is on, so the out-of-tree hypervisor can register their callbacks?
> 
> Ah, I see how I confused things with that statement.  My intent was only to call
> out that, technically, a non-NULL callback doesn't mean KVM is loaded.  I didn't
> intend to sign the kernel up for going out of its way to support out-of-tree hypervisors.

I interpret this as:

Kernel doesn't officially support the out-of-tree hypervisor, but it provides a
callback which the out-of-tree hypervisor can utilize to handle emergency virt
disable.  But such callback is only available when KVM is turned on in the
Kconfig.

?

> 
> Does it read better if I add a "that piggybacked the callback" qualifier?
> 
>   There's no need to attempt VMXOFF if KVM (or some other out-of-tree hypervisor
>   that piggybacked the callback) isn't loaded/active, i.e. if the CPU can't
>   possibly be post-VMXON. 

I think so?

But overall I just think having to consider out-of-tree hypervisor (we are
talking about a loadable module, right) only makes thing more confusing.  I
guess we can either:

1) Don't mention out-of-tree hypervisor at all.  This means kernel doesn't
officially provide mechanisms to support out-of-tree hyperivisor (a module).  If
someone wants to do that, then someone takes the risk.

2) The kernel officially provide the callback to handle emergency virt disable
for out-of-tree hypervisor (module) to use.  But this callback should be
available when KVM is off.

?