docs: document dom0less + PV drivers

Document how to use the feature and how the implementation works.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>

docs/features/dom0less.pandoc

@@ -90,6 +90,46 @@ Otherwise, they may be unusable in Xen (for instance if they are compressed).
 
 See docs/misc/arm/device-tree/booting.txt for more information.
 
+PV Drivers
+----------
+
+It is possible to use PV drivers with dom0less guests with some
+restrictions:
+
+- dom0less domUs that want to use PV drivers support should have the
+  "xen,enhanced" property set under their device tree nodes (see
+  docs/misc/arm/device-tree/booting.txt)
+- a dom0 must be present (or another domain with enough privileges to
+  run the toolstack)
+- after dom0 is booted, the utility "init-dom0less" must be run
+- do not run "init-dom0less" while creating other guests with xl
+
+After the execution of init-dom0less, it is possible to use "xl" to
+hotplug PV drivers to dom0less guests. E.g. xl network-attach domU.
+
+The implementation works as follows:
+- Xen allocates the xenstore event channel for each dom0less domU that
+  has the "xen,enhanced" property, and sets HVM_PARAM_STORE_EVTCHN
+- Xen does *not* allocate the xenstore page and sets HVM_PARAM_STORE_PFN
+  to ~0ULL (invalid)
+- Dom0less domU kernels check that HVM_PARAM_STORE_PFN is set to invalid
+    - Old kernels will continue without xenstore support (Note: some old
+      buggy kernels might crash because they don't check the validity of
+      HVM_PARAM_STORE_PFN before using it! Disable "xen,enhanced" in
+      those cases)
+    - New kernels will wait for a notification on the xenstore event
+      channel (HVM_PARAM_STORE_EVTCHN) before continuing with the
+      initialization
+- Once dom0 is booted, init-dom0less is executed:
+    - it allocates the xenstore shared page and sets HVM_PARAM_STORE_PFN
+    - it calls xs_introduce_domain
+- Xenstored notices the new domain, initializes interfaces as usual, and
+  sends an event channel notification to the domain using the xenstore
+  event channel (HVM_PARAM_STORE_EVTCHN)
+- The Linux domU kernel receives the event channel notification, checks
+  HVM_PARAM_STORE_PFN again and continue with the initialization
+
+
 Limitations
 -----------
 
@@ -107,9 +147,6 @@ limitations:
   information, the GIC version exposed to the domains started by Xen at
   boot is the same as the native GIC version.
 
-- No PV drivers. There is no support for PV devices at the moment. All
-  devices need to be statically assigned to guests.
-
 - Pinning vCPUs of domains started by Xen at boot can be
   done from the control domain, using `xl vcpu-pin` as usual. It is not
   currently possible to configure vCPU pinning without a control domain.