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@xxxxxxxxxxxxx>
> 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@xxxxxxxxxxxxx>
> ---
> 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");
>
--
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