Attempt to disable virtualization during an emergency reboot if and only
if there is a registered virt callback, i.e. iff a hypervisor (KVM) is
active. If there's no active hypervisor, then the CPU can't be operating
with VMX or SVM enabled (barring an egregious bug).
Checking for a valid callback instead of simply for SVM or VMX support
can also eliminates spurious NMIs by avoiding the unecessary call to
nmi_shootdown_cpus_on_restart().
Note, IRQs are disabled, which prevents KVM from coming along and
enabling virtualization after the fact.
Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20230721201859.2307736-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
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