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.

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);

+ 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");

On 12.03.2014 13:08, Ian Campbell wrote: > 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 ","? > Darn, I thought I had fixed that. Unfortunately in the wrong place. Yes, that should be a ",". Sorry