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fix: allow using virtiofs share as writable /nix/store overlay #419

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fix: allow using virtiofs share as writable /nix/store overlay #419

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fdcfb49
fix: allow using virtiofs share as writable /nix/store overlay
songpola Oct 12, 2025
45a21d1
refactor: use the new overlay fileSystems API
songpola Oct 13, 2025
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62 changes: 14 additions & 48 deletions nixos-modules/microvm/mounts.nix
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Original file line number Diff line number Diff line change
Expand Up @@ -27,6 +27,12 @@ let
else "/dev/vda"
else throw "No disk letter when /nix/store is not in disk";

# Check if the writable store overlay is a virtiofs share
isRwStoreVirtiofsShare = builtins.any ({mountPoint, proto, ... }:
mountPoint == config.microvm.writableStoreOverlay
&& proto == "virtiofs"
) config.microvm.shares;

in
lib.mkIf config.microvm.guest.enable {
fileSystems = lib.mkMerge [ (
Expand All @@ -53,7 +59,7 @@ lib.mkIf config.microvm.guest.enable {
) {
"/nix/store" = {
device = hostStore.mountPoint;
options = [ "bind" ];
options = [ "ro" "bind" ];
neededForBoot = true;
};
}
Expand All @@ -66,7 +72,7 @@ lib.mkIf config.microvm.guest.enable {
"/nix/.ro-store" = {
device = roStoreDisk;
fsType = storeDiskType;
options = [ "x-systemd.requires=systemd-modules-load.service" ];
options = [ "ro" "x-systemd.requires=systemd-modules-load.service" ];
neededForBoot = true;
noCheck = true;
};
Expand All @@ -75,15 +81,13 @@ lib.mkIf config.microvm.guest.enable {
# mount store with writable overlay
lib.optionalAttrs (writableStoreOverlay != null) {
"/nix/store" = {
device = "overlay";
fsType = "overlay";
neededForBoot = true;
options = [
"lowerdir=${roStore}"
"upperdir=${writableStoreOverlay}/store"
"workdir=${writableStoreOverlay}/work"
];
depends = [ roStore writableStoreOverlay ];
overlay = {
lowerdir = [ roStore ];
upperdir = "${writableStoreOverlay}/store";
workdir = "${writableStoreOverlay}/work";
};
options = lib.optional isRwStoreVirtiofsShare "userxattr";
};
}
) {
Expand Down Expand Up @@ -125,42 +129,4 @@ lib.mkIf config.microvm.guest.enable {
};
}) {} config.microvm.shares
) ];

# boot.initrd.systemd patchups copied from <nixpkgs/nixos/modules/virtualisation/qemu-vm.nix>
boot.initrd.systemd = lib.mkIf (config.boot.initrd.systemd.enable && writableStoreOverlay != null) {
mounts = [ {
where = "/sysroot/nix/store";
what = "overlay";
type = "overlay";
options = builtins.concatStringsSep "," [
"lowerdir=/sysroot${roStore}"
"upperdir=/sysroot${writableStoreOverlay}/store"
"workdir=/sysroot${writableStoreOverlay}/work"
];
wantedBy = [ "initrd-fs.target" ];
before = [ "initrd-fs.target" ];
requires = [ "rw-store.service" ];
after = [ "rw-store.service" ];
unitConfig.RequiresMountsFor = "/sysroot/${roStore}";
} ];
services.rw-store = {
unitConfig = {
DefaultDependencies = false;
RequiresMountsFor = "/sysroot${writableStoreOverlay}";
};
serviceConfig = {
Type = "oneshot";
ExecStart = "/bin/mkdir -p -m 0755 /sysroot${writableStoreOverlay}/store /sysroot${writableStoreOverlay}/work /sysroot/nix/store";
};
};
};

# Fix for hanging shutdown
systemd.mounts = lib.mkIf config.boot.initrd.systemd.enable [ {
what = "store";
where = "/nix/store";
# Generate a `nix-store.mount.d/overrides.conf`
overrideStrategy = "asDropin";
unitConfig.DefaultDependencies = false;
} ];
}