Re: [libvirt] libvirt: [PATCH] libxl: Check for control_d string to decide about dom0

On Wed, 2014-03-12 at 13:03 +0100, Stefan Bader wrote: > I have been looking into a bug report (see BugLink) which reported > libvirt to fail starting inside a Xen guest. Upon further investigation > I found that some tools that help monitoring Xen guests will mount > xenfs to /proc/xen. This will create a capabilities files there even > if the guest is not dom0. However it will return nothing when reading > from it. This seems consistent with the xencommons initscript which does: # run this script only in dom0: # no capabilities file in xenlinux domU kernel # empty capabilities file in pv_ops domU kernel if test -f /proc/xen/capabilities && \ ! grep -q "control_d" /proc/xen/capabilities ; then exit 0 fi > > Ian, just to sanity check myself. I looked at the xenfs code and to > me there only seem to be those two outcomes (either "control_d" for > running in dom0 or notrhing if not). > > With the following patch applied, libvirt starts up correctly in > the normal guests (with xenfs mounted) without initializing libxl. > And also in dom0 where it still enables the libxl driver (if the > xl toolstack is selected). > > -Stefan > > From f11949caca6dfe1a802472a2a6d4fe760115ccc6 Mon Sep 17 00:00:00 2001 > From: Stefan Bader <stefan.bader(a)canonical.com&gt; > Date: Wed, 12 Mar 2014 11:37:16 +0100 > Subject: [PATCH] libxl: Check for control_d string to decide about dom0 > > As soon as any guest mounts xenfs to /proc/xen, there is a capabilities > file in that directory. However it returns nothing when reading from it. > Change the test to actually check the contents of the file. > > BugLink: http://bugs.launchpad.net/bugs/1248025 > > Signed-off-by: Stefan Bader <stefan.bader(a)canonical.com&gt; > --- > src/libxl/libxl_driver.c | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) > > diff --git a/src/libxl/libxl_driver.c b/src/libxl/libxl_driver.c > index 65d80a2..844e828 100644 > --- a/src/libxl/libxl_driver.c > +++ b/src/libxl/libxl_driver.c > @@ -944,6 +944,7 @@ libxlDriverShouldLoad(bool privileged) > bool ret = false; > virCommandPtr cmd; > int status; > + char *output = NULL; > > /* Don't load if non-root */ > if (!privileged) { > @@ -951,8 +952,17 @@ libxlDriverShouldLoad(bool privileged) > return ret; > } > > - /* Don't load if not running on a Xen control domain (dom0) */ > - if (!virFileExists("/proc/xen/capabilities")) { > + /* > + * Don't load if not running on a Xen control domain (dom0). It is not > + * sufficient to check for the file to exist as any guest can mount > + * xenfs to /proc/xen. > + */ > + status = virFileReadAll("/proc/xen/capabilities", 10. &output); Is this "." supposed to be a ","?

> + if (status >= 0) { > + status = strncmp(output, "control_d", 9); > + } > + VIR_FREE(output); > + if (status) { > VIR_INFO("No Xen capabilities detected, probably not running " > "in a Xen Dom0. Disabling libxenlight driver"); >