qemu: emit warnings when KVM acceleration is not usable

This fixes the qemu-kvm wrapper we add for convenience
silently not using KVM, when the system would support it
by at least leaving an indication in the log that the build ran
slower because it ran without KVM.

pkgs/applications/virtualization/qemu/default.nix

@@ -1,7 +1,7 @@
 { lib, stdenv, fetchurl, fetchpatch, python, zlib, pkg-config, glib
 , perl, pixman, vde2, alsa-lib, texinfo, flex
 , bison, lzo, snappy, libaio, libtasn1, gnutls, nettle, curl, ninja, meson, sigtool
-, makeWrapper, autoPatchelfHook
+, makeWrapper, autoPatchelfHook, runtimeShell
 , attr, libcap, libcap_ng
 , CoreServices, Cocoa, Hypervisor, rez, setfile
 , numaSupport ? stdenv.isLinux && !stdenv.isAarch32, numactl
@@ -228,9 +228,12 @@ stdenv.mkDerivation rec {
 
   # Add a ‘qemu-kvm’ wrapper for compatibility/convenience.
   postInstall = ''
+    cp -- $emitKvmWarningsPath $out/libexec/emit-kvm-warnings
+    chmod a+x -- $out/libexec/emit-kvm-warnings
     if [ -x $out/bin/qemu-system-${stdenv.hostPlatform.qemuArch} ]; then
       makeWrapper $out/bin/qemu-system-${stdenv.hostPlatform.qemuArch} \
                   $out/bin/qemu-kvm \
+                  --run $out/libexec/emit-kvm-warnings \
                   --add-flags "\$([ -r /dev/kvm -a -w /dev/kvm ] && echo -enable-kvm)"
     fi
   '';
@@ -242,6 +245,26 @@ stdenv.mkDerivation rec {
   # Builds in ~3h with 2 cores, and ~20m with a big-parallel builder.
   requiredSystemFeatures = [ "big-parallel" ];
 
+  emitKvmWarnings = ''
+    #!${runtimeShell}
+    WARNCOL='\033[1;35m'
+    NEUTRALCOL='\033[0m'
+    WARNING="''${WARNCOL}warning:''${NEUTRALCOL}"
+    if [ ! -e /dev/kvm ]; then
+      echo -e "''${WARNING} KVM is not available - execution will be slow" >&2
+      echo "Consider installing KVM for hardware-accelerated execution." >&2
+      echo "If KVM is already installed make sure the kernel module is loaded." >&2
+    elif [ ! -r /dev/kvm -o ! -w /dev/kvm ]; then
+      echo -e "''${WARNING} /dev/kvm is not read-/writable - execution will be slow" >&2
+      echo "/dev/kvm needs to be read-/writable by the user executing QEMU." >&2
+      echo "" >&2
+      echo "For hardware-acceleration inside the nix build sandbox /dev/kvm" >&2
+      echo "must be world-read-/writable (rw-rw-rw-)." >&2
+    fi
+  '';
+
+  passAsFile = [ "emitKvmWarnings" ];
+
   meta = with lib; {
     homepage = "http://www.qemu.org/";
     description = "A generic and open source machine emulator and virtualizer";