From d85dc4f6902819be8277d1d65414e51bfbef0ac6 Mon Sep 17 00:00:00 2001
From: Erik Arvstedt <erik.arvstedt@gmail.com>
Date: Wed, 27 May 2020 22:07:43 +0200
Subject: [PATCH] qemu-vm: fix inconsistent caching of xchg dirs

xchg is advertised as a bidirectional exchange dir, but file content
transfer from host to VM fails due to caching:
If a file is read in the VM and then modified on the host, subsequent
re-reads in the VM can yield old, cached data.
This is caused by the use of 9p's cache=loose mode that is explicitly
meant for read-only mounts.

9p doesn't provide any suitable cache modes, so fix this by disabling
caching.

Also, remove a now unnecessary sync in the test driver.
---
 nixos/lib/test-driver/test-driver.py                 | 1 -
 nixos/modules/virtualisation/qemu-vm.nix             | 2 +-
 pkgs/build-support/vm/default.nix                    | 2 +-
 pkgs/build-support/vm/windows/controller/default.nix | 2 +-
 4 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/nixos/lib/test-driver/test-driver.py b/nixos/lib/test-driver/test-driver.py
index ff6d763989f5..b96165823721 100644
--- a/nixos/lib/test-driver/test-driver.py
+++ b/nixos/lib/test-driver/test-driver.py
@@ -615,7 +615,6 @@ class Machine:
             # Copy the file to the shared directory inside VM
             self.succeed(make_command(["mkdir", "-p", vm_shared_temp]))
             self.succeed(make_command(["cp", "-r", vm_src, vm_intermediate]))
-            self.succeed("sync")
             abs_target = out_dir / target_dir / vm_src.name
             abs_target.parent.mkdir(exist_ok=True, parents=True)
             # Copy the file from the shared directory outside VM
diff --git a/nixos/modules/virtualisation/qemu-vm.nix b/nixos/modules/virtualisation/qemu-vm.nix
index 31d332e9f07a..3ecb8aa11ca3 100644
--- a/nixos/modules/virtualisation/qemu-vm.nix
+++ b/nixos/modules/virtualisation/qemu-vm.nix
@@ -567,7 +567,7 @@ in
         "/tmp/xchg" =
           { device = "xchg";
             fsType = "9p";
-            options = [ "trans=virtio" "version=9p2000.L" "cache=loose" ];
+            options = [ "trans=virtio" "version=9p2000.L" ];
             neededForBoot = true;
           };
         "/tmp/shared" =
diff --git a/pkgs/build-support/vm/default.nix b/pkgs/build-support/vm/default.nix
index 488c7f2ce2c7..cdd713be180d 100644
--- a/pkgs/build-support/vm/default.nix
+++ b/pkgs/build-support/vm/default.nix
@@ -115,7 +115,7 @@ rec {
 
     echo "mounting host's temporary directory..."
     mkdir -p /fs/tmp/xchg
-    mount -t 9p xchg /fs/tmp/xchg -o trans=virtio,version=9p2000.L,cache=loose
+    mount -t 9p xchg /fs/tmp/xchg -o trans=virtio,version=9p2000.L
 
     mkdir -p /fs/proc
     mount -t proc none /fs/proc
diff --git a/pkgs/build-support/vm/windows/controller/default.nix b/pkgs/build-support/vm/windows/controller/default.nix
index 08b93aaf1174..9d13983a2833 100644
--- a/pkgs/build-support/vm/windows/controller/default.nix
+++ b/pkgs/build-support/vm/windows/controller/default.nix
@@ -53,7 +53,7 @@ let
       store /fs/nix/store
 
     mount -t 9p \
-      -o trans=virtio,version=9p2000.L,cache=loose \
+      -o trans=virtio,version=9p2000.L \
       xchg /fs/xchg
 
     echo root:x:0:0::/root:/bin/false > /fs/etc/passwd