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Date: Tue, 21 May 2024 16:16:46 -0700
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Subject: Re: [PATCH 1/4] x86/reboot: Unconditionally define
 cpu_emergency_virt_cb typedef
From: Sean Christopherson <seanjc@google.com>
To: Kai Huang <kai.huang@intel.com>
Cc: "pbonzini@redhat.com" <pbonzini@redhat.com>, "kvm@vger.kernel.org" <kvm@vger.kernel.org>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Wed, May 22, 2024, Kai Huang wrote:
> On 22/05/2024 8:02 am, Sean Christopherson wrote:
> > On Wed, May 15, 2024, Kai Huang wrote:
> > > How about we just make all emergency virtualization disable code
> > > unconditional but not guided by CONFIG_KVM_INTEL || CONFIG_KVM_AMD, i.e.,
> > > revert commit
> > > 
> > >     261cd5ed934e ("x86/reboot: Expose VMCS crash hooks if and only if
> > > KVM_{INTEL,AMD} is enabled")
> > > 
> > > It makes sense anyway from the perspective that it allows the out-of-tree
> > > kernel module hypervisor to use this mechanism w/o needing to have the
> > > kernel built with KVM enabled in Kconfig.  Otherwise, strictly speaking,
> > > IIUC, the kernel won't be able to support out-of-tree module hypervisor as
> > > there's no other way the module can intercept emergency reboot.
> > 
> > Practically speaking, no one is running an out-of-tree hypervisor without either
> > (a) KVM being enabled in the .config, or (b) non-trivial changes to the kernel.
> 
> Just for curiosity: why b) is required to support out-of-tree hypervisor
> when KVM is disabled in Kconfig?  I am probably missing something.

A variety of hooks that are likely needed for any x86 hypervisor (especially on
Intel) are guarded by CONFIG_KVM.  E.g. the posted interrupt vectors (though it's
definitely possible to use a different model than KVM), the entry point for
trampolining NMIs (though again, a hypervisor could just do "INT 2", at least
until FRED comes along), and probably a few other things.

I'm sure it's possible to workaround any issues, but I would be quite surprised
if out-of-tree code went through the effort of functioning with a kernel built
to play nice with KVM.  Who knows, maybe I'm entirely wrong :-)