Merge pull request #142577 from helsinki-systems/feat/larger-p9-msize

vmTools: Make msize larger to silence warning

nixos/modules/virtualisation/qemu-vm.nix

···
     virtualisation.msize =
       mkOption {
         type = types.ints.positive;
-        default = 16384;
+        default = pkgs.vmTools.default9PMsizeBytes;
         description =
           ''
             The msize (maximum packet size) option passed to 9p file systems, in

pkgs/build-support/vm/default.nix

···
 
     echo "mounting Nix store..."
     mkdir -p /fs${storeDir}
-    mount -t 9p store /fs${storeDir} -o trans=virtio,version=9p2000.L,cache=loose
+    mount -t 9p store /fs${storeDir} -o trans=virtio,version=9p2000.L,cache=loose,msize=${toString default9PMsizeBytes}
 
     mkdir -p /fs/tmp /fs/run /fs/var
     mount -t tmpfs -o "mode=1777" none /fs/tmp
···
 
     echo "mounting host's temporary directory..."
     mkdir -p /fs/tmp/xchg
-    mount -t 9p xchg /fs/tmp/xchg -o trans=virtio,version=9p2000.L
+    mount -t 9p xchg /fs/tmp/xchg -o trans=virtio,version=9p2000.L,msize=${toString default9PMsizeBytes}
 
     mkdir -p /fs/proc
     mount -t proc none /fs/proc
···
      `debDistros' sets. */
   diskImages = lib.mapAttrs (name: f: f {}) diskImageFuns;
 
+  # The default 9P msize value is 8 KiB, which according to QEMU is
+  # insufficient and would degrade performance.
+  # See: https://wiki.qemu.org/Documentation/9psetup#msize
+  # Use 500 KiB as a conservative default, see also https://github.com/NixOS/nixpkgs/pull/142577#issuecomment-953848731
+  default9PMsizeBytes = 512000;
 }