Re: [PATCH v2 05/18] x86/reboot: KVM: Disable SVM during reboot via virt/KVM reboot callback

[Sean Christopherson <seanjc@google.com>]

On Mon, Mar 13, 2023, Huang, Kai wrote:
> On Fri, 2023-03-10 at 13:42 -0800, Sean Christopherson wrote:
> > Use the virt callback to disable SVM (and set GIF=1) during an emergency
> > instead of blindly attempting to disable SVM.� Like the VMX case, if KVM
> > (or an out-of-tree hypervisor) isn't loaded/active, SVM can't be in use.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> 
> [...]
> 
> > -#if IS_ENABLED(CONFIG_KVM_INTEL)
> > +#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
> > �/* RCU-protected callback to disable virtualization prior to reboot. */
> > �static cpu_emergency_virt_cb __rcu *cpu_emergency_virt_callback;
> > �
> > @@ -821,7 +821,7 @@ int crashing_cpu = -1;
> > � */
> > �void cpu_emergency_disable_virtualization(void)
> > �{
> > -#if IS_ENABLED(CONFIG_KVM_INTEL)
> > +#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
> > �	cpu_emergency_virt_cb *callback;
> > �
> > �	rcu_read_lock();
> > @@ -830,8 +830,6 @@ void cpu_emergency_disable_virtualization(void)
> > �		callback();
> > �	rcu_read_unlock();
> > �#endif
> > -	/* KVM_AMD doesn't yet utilize the common callback. */
> > -	cpu_emergency_svm_disable();
> > �}
> 
> Shouldn't the callback be always present since you want to consider 'out-of-
> tree' hypervisor case?

No?  The kernel doesn't provide any guarantees for out-of-tree code.  I don't have
a super strong preference, though I do like the effective documentation the checks
provide.  Buy more importantly, my understanding is that the x86 maintainers want
to limit the exposure for these types of interfaces, e.g. `git grep IS_ENABLED\(CONFIG_KVM`
for a variety of hooks that are enabled iff KVM is enabled in the kernel config.