[PATCH v3 8/8] xen/dom0less: store xenstore event channel in page

[Jason Andryuk <jason.andryuk@xxxxxxx>]

Write the associated event channel into the xenstore page so xenstored
can read it.  xenstored can map the grant by the reserved grant table
entry, and then read out the event channel and bind it.  This eliminates
the need for an additional mechanism to discover the event channel.

Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
---
v2:
No change

This should go in after the init-dom0less changes so init-dom0less is
ready for xenstored automatically introducing domains.

I'm looking for feedback.  This is ARM-only for the time being, but that
is the only in-tree user of this code.  From the perspective, it is okay
to go in.

If we want a cross-arch approach, a common function to write to guest
physical addresses would be needed for additional arches, but they
aren't available yet.

Oleksii added a function pointer to dtb_load() and initrd_load() when
moving dom0less to common, but I think that isn't necessary.  Just
having a common helper would be sufficient.

copy_to_guest_phys() or something_copy_to_guest_phys() could be defined
or a wrapper for ARM's copy_to_guest_phys_flush_dcache().  Other arches
could need to implement it when using dom0less.

I'm not an ARM expert, but Stefano said
copy_to_guest_phys_flush_dcache() is not necessary since this xenstore
page isn't expected to be accessed without caches enabled.
---
 xen/common/device-tree/dom0less-build.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/xen/common/device-tree/dom0less-build.c 
b/xen/common/device-tree/dom0less-build.c
index badc227031..1a40f68837 100644
--- a/xen/common/device-tree/dom0less-build.c
+++ b/xen/common/device-tree/dom0less-build.c
@@ -26,6 +26,7 @@
 #include <public/event_channel.h>
 #include <public/io/xs_wire.h>
 
+#include <asm/guest_access.h>
 #include <asm/setup.h>
 
 #include <xen/static-memory.h>
@@ -120,8 +121,14 @@ static void __init initialize_domU_xenstore(void)
 
         if ( gfn != XENSTORE_PFN_LATE_ALLOC && IS_ENABLED(CONFIG_GRANT_TABLE) )
         {
+            evtchn_port_t port = d->arch.hvm.params[HVM_PARAM_STORE_EVTCHN];
+            paddr_t evtchn_gaddr = gfn_to_gaddr(_gfn(gfn)) +
+                offsetof(struct xenstore_domain_interface, evtchn_port);
+
             ASSERT(gfn < UINT32_MAX);
             gnttab_seed_entry(d, GNTTAB_RESERVED_XENSTORE, xs_domid, gfn);
+            access_guest_memory_by_gpa(d, evtchn_gaddr, &port, sizeof(port),
+                                       true /* is_write */);
         }
     }
 }
-- 
2.50.1