From 0390e69d8d8569776ceefd5ec424eb8e21dfed85 Mon Sep 17 00:00:00 2001
From: Jeremy Fleischman <me@jfly.fyi>
Date: Mon, 14 Sep 2026 15:26:22 -0500
Subject: [PATCH] nixos/qemu-vm: virtiofsd requires shared memory

We recently switched to virtiofs for qemu-vms run on Linux
(https://github.com/NixOS/nixpkgs/pull/552774). I suspect we
only tested it with nixos tests, which just [happen to enable shared memory](https://github.com/NixOS/nixpkgs/blob/673028b7b1845fb74dab830518892c917634b95a/nixos/lib/testing/driver.nix#L259)
for the ssh backdoor. However, for folks using qemu-vm.nix outside of nixos
tests, they'll experience virtiofsd crashing with [some very obtuse
messages](https://gitlab.com/virtio-fs/virtiofsd/-/work_items/127).

Reported by @fricklerhandwerk in
<https://github.com/NixOS/nixpkgs/pull/552774#issuecomment-5668027063>.
---
 nixos/modules/virtualisation/qemu-vm.nix | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/nixos/modules/virtualisation/qemu-vm.nix b/nixos/modules/virtualisation/qemu-vm.nix
index 61366c3ea75f..3afe34f0a2a7 100644
--- a/nixos/modules/virtualisation/qemu-vm.nix
+++ b/nixos/modules/virtualisation/qemu-vm.nix
@@ -741,7 +741,11 @@ in
     };
 
     virtualisation.qemu = {
-      enableSharedMemory = mkEnableOption "shared memory";
+      enableSharedMemory = mkOption {
+        type = types.bool;
+        default = useVirtiofs; # Need shared memory for virtiofs: <https://www.qemu.org/docs/master/system/devices/virtio/vhost-user.html#shared-memory-object>
+        description = "Enable shared memory";
+      };
 
       package = mkOption {
         type = types.package;