VMX: don't crash processing 'd' debug key

VMX: don't crash processing 'd' debug key

There's a window during scheduling where "current" and the active VMCS
may disagree: The former gets set much earlier than the latter. Since
both vmx_vmcs_enter() and vmx_vmcs_exit() immediately return when the
subject vCPU is "current", accessing VMCS fields would, depending on
whether there is any currently active VMCS, either read wrong data, or
cause a crash.

Going forward we might want to consider reducing the window during
which vmx_vmcs_enter() might fail (e.g. doing a plain __vmptrld() when
v->arch.hvm_vmx.vmcs != this_cpu(current_vmcs) but arch_vmx->active_cpu
== -1), but that would add complexities (acquiring and - more
importantly - properly dropping v->arch.hvm_vmx.vmcs_lock) that don't
look worthwhile adding right now.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>