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[Libguestfs] [PATCH] Add -enable-kvm option to qemu command line.

Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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>From 245ac5673700f54479e9408e8659cb68f80c344a Mon Sep 17 00:00:00 2001
From: Richard Jones <rjones redhat com>
Date: Tue, 9 Feb 2010 16:30:18 +0000
Subject: [PATCH] Add -enable-kvm option to qemu command line.

If the version of qemu being used supports -enable-kvm option,
then check if /dev/kvm is openable and add this option.

I have found this option makes no difference, although it is
*supposed* to enable KVM (hardware virtualization) support.
 src/guestfs.c |   31 +++++++++++++++++++++++++++++++
 1 files changed, 31 insertions(+), 0 deletions(-)

diff --git a/src/guestfs.c b/src/guestfs.c
index 1d6d40b..9908e7f 100644
--- a/src/guestfs.c
+++ b/src/guestfs.c
@@ -858,6 +858,7 @@ static void print_timestamped_message (guestfs_h *g, const char *fs, ...);
 static int build_supermin_appliance (guestfs_h *g, const char *path, char **kernel, char **initrd);
 static int test_qemu (guestfs_h *g);
 static int qemu_supports (guestfs_h *g, const char *option);
+static int is_openable (guestfs_h *g, const char *path, int flags);
 static void print_cmdline (guestfs_h *g);
 static const char *kernel_name = "vmlinuz." REPO "." host_cpu;
@@ -1091,6 +1092,22 @@ guestfs__launch (guestfs_h *g)
     g->cmdline[0] = g->qemu;
+    /* qemu sometimes needs this option to enable hardware
+     * virtualization, but some versions of 'qemu-kvm' will use KVM
+     * regardless (even where this option appears in the help text).
+     * It is rumoured that there are versions of qemu where supplying
+     * this option when hardware virtualization is not available will
+     * cause qemu to fail, so we we have to check at least that
+     * /dev/kvm is openable.  That's not reliable, since /dev/kvm
+     * might be openable by qemu but not by us (think: SELinux) in
+     * which case the user would not get hardware virtualization,
+     * although at least shouldn't fail.  A giant clusterfuck with the
+     * qemu command line, again.
+     */
+    if (qemu_supports (g, "-enable-kvm") &&
+        is_openable (g, "/dev/kvm", O_RDWR))
+      add_cmdline (g, "-enable-kvm");
     /* Newer versions of qemu (from around 2009/12) changed the
      * behaviour of monitors so that an implicit '-monitor stdio' is
      * assumed if we are in -nographic mode and there is no other
@@ -1647,6 +1664,20 @@ qemu_supports (guestfs_h *g, const char *option)
   return g->qemu_help && strstr (g->qemu_help, option) != NULL;
+/* Check if a file can be opened. */
+static int
+is_openable (guestfs_h *g, const char *path, int flags)
+  int fd = open (path, flags);
+  if (fd == -1) {
+    if (g->verbose)
+      perror (path);
+    return 0;
+  }
+  close (fd);
+  return 1;
 /* Check the peer effective UID for a TCP socket.  Ideally we'd like
  * SO_PEERCRED for a loopback TCP socket.  This isn't possible on
  * Linux (but it is on Solaris!) so we read /proc/net/tcp instead.

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