Re: [Xen-devel] [PATCH v1] tools/hotplug: convert proc-xen.mount to proc-xen.service

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Oct 26, 2017 at 04:45:38PM +0100, Andrew Cooper wrote:
> On 26/10/17 16:25, Olaf Hering wrote:
> > An upcoming change in systemd will mount xenfs right away, along with
> > all other system mounts. This improves the detection of the
> > virtualization environment, which is currently racy. Some parts of
> > systemd rely on the presence of /proc/xen/capabilities, which will only
> > exist if xenfs is mounted. Since xenfs is mounted by the proc-xen.mount
> > unit, it will be processed very late. Other units may be processed
> > earlier, and if they make use of ConditionVirtualization*= failures may
> > occour.
> >
> > Unfortunately mounting xenfs by systemd as an API filesystem will lead
> > to errors when proc-xen.mount is processed. Since that mount point
> > already exists the unit is considered as failed, and other units that
> > depend on proc-xen.mount will not start. To avoid this the existing
> > proc-xen.mount will be converted into proc-xen.service, which just
> > mounts xenfs manually. All dependencies are updated by this change.
> >
> > The existing conditionals in proc-xen.mount will prevent failures with
> > existing systemd based installations:
> > ConditionPathExists=!/proc/xen/capabilities will prevent execution with
> > a new systemd that mounts xenfs. And this conditional, in combination
> > with ConditionPathExists=/proc/xen, will trigger execution with an old
> > systemd.
> >
> > An absolute path to the mount binary has to be used. /bin/mount is
> > expected to be universally available, nowaways it is a symlink to
> > /usr/bin/mount.
> >
> > Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>
> 
> Can't all information be obtained from /sys/hypervisor?  If not, how
> hard would it be to make happen?

It is not possible to tell from /sys/hypervisor at the moment.

There is /sys/hypervisor/properties/features but that only contains
feature bits from hypervisor. And XENFEAT_dom0 is not a reliable
indicator for Dom0.

I suppose it requires a small mount of work to make /sys/hypervisor
contain such information, but that won't get propagated to older kernel
so we need to deal with the problem at hand nonetheless.

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