x86_64: enable nested virtualization (speculatively)

hosts/modules/hal/x86_64.nix

@@ -27,8 +27,16 @@ in
       "rtc_cmos"
     ];
 
-    hardware.cpu.amd.updateMicrocode = true;    # desktop
-    hardware.cpu.intel.updateMicrocode = true;  # laptop
+    hardware.cpu.amd.updateMicrocode = true;    # desko
+    hardware.cpu.intel.updateMicrocode = true;  # lappy
+
+    boot.extraModprobeConfig = ''
+      # allow nested virtualization. XXX(2025-06-24): required for my work (?)
+      # see: <https://wiki.nixos.org/wiki/Libvirt#Nested_virtualization>
+      # see: <https://www.linux-kvm.org/page/Nested_Guests>
+      options kvm_amd nested=1
+      options kvm_intel nested=1
+    '';
 
     sane.image.extraBootFiles = [ pkgs.bootpart-uefi-x86_64 ];
   };