nixos/libvirt: avoid dependency on two qemu packages

Currently libvirt requires two qemu derivations: qemu and qemu_kvm which is just a truncated version of qemu (defined as qemu.override { hostCpuOnly = true; }).

This patch exposes an option virtualisation.libvirtd.qemuPackage which allows to choose which package to use:

 * pkgs.qemu_kvm if all your guests have the same CPU as host, or
 * pkgs.qemu which allows to emulate alien architectures (for example ARMV7L on X86_64), or
 * a custom derivation

virtualisation.libvirtd.enableKVM option is vague and could be deprecate in favor of virtualisation.libvirtd.qemuPackage, anyway it does allow to enable/disable kvm.

2 files changed, 16 insertions, 9 deletions

diff --git a/nixos/modules/virtualisation/libvirtd.nix b/nixos/modules/virtualisation/libvirtd.nix
index 8aa7ad8e3911..7208b23116a5 100644
--- a/nixos/modules/virtualisation/libvirtd.nix
+++ b/nixos/modules/virtualisation/libvirtd.nix
@@ -41,7 +41,16 @@ in {
       type = types.bool;
       default = true;
       description = ''
-        This option enables support for QEMU/KVM in libvirtd.
+        This option disables support for non-KVM guests in libvirtd (e.g. aarch64 on x86).
+        KVM is available even if this setting is false.
+      '';
+    };
+
+    virtualisation.libvirtd.qemuPackage = mkOption {
+      type = types.package;
+      default = if cfg.enableKVM then pkgs.qemu_kvm else pkgs.qemu;
+      description = ''
+        Qemu package to use with libvirt
       '';
     };
 
@@ -102,7 +111,7 @@ in {
 
   config = mkIf cfg.enable {
 
-    environment.systemPackages = with pkgs; [ libvirt netcat-openbsd qemu_kvm ];
+    environment.systemPackages = with pkgs; [ libvirt netcat-openbsd cfg.qemuPackage ];
 
     boot.kernelModules = [ "tun" ];
 
@@ -154,9 +163,9 @@ in {
 
         # stable (not GC'able as in /nix/store) paths for using in <emulator> section of xml configs
         mkdir -p /run/libvirt/nix-emulators
-        ln -s --force ${pkgs.libvirt}/libexec/libvirt_lxc /run/libvirt/nix-emulators/
-        ${optionalString pkgs.stdenv.isAarch64 "ln -s --force ${pkgs.qemu}/bin/qemu-system-aarch64 /run/libvirt/nix-emulators/"}
-        ${optionalString cfg.enableKVM         "ln -s --force ${pkgs.qemu_kvm}/bin/qemu-kvm        /run/libvirt/nix-emulators/"}
+        for emulator in ${pkgs.libvirt}/libexec/libvirt_lxc ${cfg.qemuPackage}/bin/qemu-kvm ${cfg.qemuPackage}/bin/qemu-system-*; do
+          ln -s --force "$emulator" /run/libvirt/nix-emulators/
+        done
 
         ${optionalString cfg.qemuOvmf ''
             mkdir -p /run/libvirt/nix-ovmf
diff --git a/pkgs/development/libraries/libvirt/default.nix b/pkgs/development/libraries/libvirt/default.nix
index 754588504160..2dbf80e71f77 100644
--- a/pkgs/development/libraries/libvirt/default.nix
+++ b/pkgs/development/libraries/libvirt/default.nix
@@ -4,7 +4,7 @@
 , iproute, iptables, readline, lvm2, utillinux, systemd, libpciaccess, gettext
 , libtasn1, ebtables, libgcrypt, yajl, pmutils, libcap_ng, libapparmor
 , dnsmasq, libnl, libpcap, libxslt, xhtml1, numad, numactl, perlPackages
-, curl, libiconv, gmp, xen, zfs, parted, qemu
+, curl, libiconv, gmp, xen, zfs, parted
 }:
 
 with stdenv.lib;
@@ -41,8 +41,6 @@ stdenv.mkDerivation rec {
 
     # the path to qemu-kvm will be stored in VM's .xml and .save files
     # do not use "''${qemu_kvm}/bin/qemu-kvm" to avoid bound VMs to particular qemu derivations
-    substituteInPlace src/qemu/qemu_capabilities.c \
-      --replace '"/usr/libexec/qemu-kvm"' '"/run/libvirt/nix-emulators/${if stdenv.isAarch64 then "qemu-system-aarch64" else "qemu-kvm"}"'
     substituteInPlace src/lxc/lxc_conf.c \
       --replace 'lxc_path,' '"/run/libvirt/nix-emulators/libvirt_lxc",'
   '' + ''
@@ -91,7 +89,7 @@ stdenv.mkDerivation rec {
     substituteInPlace $out/lib/systemd/system/libvirtd.service --replace /bin/kill ${coreutils}/bin/kill
     rm $out/lib/systemd/system/{virtlockd,virtlogd}.*
     wrapProgram $out/sbin/libvirtd \
-      --prefix PATH : ${makeBinPath [ iptables iproute pmutils numad numactl qemu ]}
+      --prefix PATH : /run/libvirt/nix-emulators:${makeBinPath [ iptables iproute pmutils numad numactl ]}
   '';
 
   enableParallelBuilding = true;