drivers/xen/hypervisor: Expose Xen SIF flags to userspace

/proc/xen is a legacy pseudo filesystem which predates Xen support
getting merged into Linux. It has largely been replaced with more
normal locations for data (/sys/hypervisor/ for info, /dev/xen/ for
user devices). We want to compile xenfs support out of the dom0 kernel.

There is one item which only exists in /proc/xen, namely
/proc/xen/capabilities with "control_d" being the signal of "you're in
the control domain". This ultimately comes from the SIF flags provided
at VM start.

This patch exposes all SIF flags in /sys/hypervisor/start_flags/ as
boolean files, one for each bit, returning '1' if set, '0' otherwise.
Two known flags, 'privileged' and 'initdomain', are explicitly named,
and all remaining flags can be accessed via generically named files,
as suggested by Andrew Cooper.

Signed-off-by: Per Bilse <per.bilse@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20230103130213.2129753-1-per.bilse@citrix.com
Signed-off-by: Juergen Gross <jgross@suse.com>

docs: ABI: stable: make files ReST compatible

Several entries at the stable ABI files won't parse if we pass
them directly to the ReST output.

Adjust them, in order to allow adding their contents as-is at
the stable ABI book.

Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/467a0dfbcdf00db710a629d3fe4a2563750339d8.1604042072.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

doc,xen: document hypervisor sysfs nodes for xen

Today only a few sysfs nodes under /sys/hypervisor/ are documented
for Xen in Documentation/ABI/testing/sysfs-hypervisor-pmu.

Add the remaining Xen sysfs nodes under /sys/hypervisor/ in a new
file Documentation/ABI/stable/sysfs-hypervisor-xen and add the Xen
specific sysfs docs to the MAINTAINERS file.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>