Re: [PATCH v2 6/6] KVM: x86: Register "emergency disable" callbacks when virt is enabled

["Huang, Kai" <kai.huang@intel.com>]

On 22/05/2024 2:28 pm, Sean Christopherson wrote:
> Register the "disable virtualization in an emergency" callback just
> before KVM enables virtualization in hardware, as there is no functional
> need to keep the callbacks registered while KVM happens to be loaded, but
> is inactive, i.e. if KVM hasn't enabled virtualization.
> 
> Note, unregistering the callback every time the last VM is destroyed could
> have measurable latency due to the synchronize_rcu() needed to ensure all
> references to the callback are dropped before KVM is unloaded.  But the
> latency should be a small fraction of the total latency of disabling
> virtualization across all CPUs, and userspace can set enable_virt_at_load
> to completely eliminate the runtime overhead.
> 
> Add a pointer in kvm_x86_ops to allow vendor code to provide its callback.
> There is no reason to force vendor code to do the registration, and either
> way KVM would need a new kvm_x86_ops hook.
> 
> Suggested-by: Kai Huang <kai.huang@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---

Reviewed-by: Kai Huang <kai.huang@intel.com>