qemu, runInLinuxVM: fix KVM availability check

nixos/lib/qemu-common.nix

@@ -22,7 +22,7 @@ rec {
         else throw "Unknown QEMU serial device for system '${pkgs.stdenv.hostPlatform.system}'";
 
   qemuBinary = qemuPkg: {
-    x86_64-linux = "${qemuPkg}/bin/qemu-kvm -cpu max";
+    x86_64-linux = "${qemuPkg}/bin/qemu-kvm -cpu qemu64";
     armv7l-linux = "${qemuPkg}/bin/qemu-system-arm -enable-kvm -machine virt -cpu host";
     aarch64-linux = "${qemuPkg}/bin/qemu-system-aarch64 -enable-kvm -machine virt,gic-version=host -cpu host";
     powerpc64le-linux = "${qemuPkg}/bin/qemu-system-ppc64 -machine powernv";

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
@@ -219,10 +219,13 @@ 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 \
-                  --add-flags "\$([ -e /dev/kvm ] && echo -enable-kvm)"
+                  --run $out/libexec/emit-kvm-warnings \
+                  --add-flags "\$([ -r /dev/kvm -a -w /dev/kvm ] && echo -enable-kvm)"
     fi
   '';
 
@@ -233,6 +236,26 @@ 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";