[PATCH 3/3][4.17?] x86/pv-shim: correct ballooning down for compat guests

[Jan Beulich <jbeulich@xxxxxxxx>]

From: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>

The compat layer for multi-extent memory ops may need to split incoming
requests. Since the guest handles in the interface structures may not be
altered, it does so by leveraging do_memory_op()'s continuation
handling: It hands on non-initial requests with a non-zero start extent,
with the (native) handle suitably adjusted down. As a result
do_memory_op() sees only the first of potentially several requests with
start extent being zero. In order to be usable as overall result, the
function accumulates args.nr_done, i.e. it initialized the field with
the start extent. Therefore non-initial requests resulting from the
split would pass too large a number into pv_shim_offline_memory().

Address that breakage by always calling pv_shim_offline_memory()
regardless of current hypercall preemption status, with a suitably
adjusted first argument. Note that this is correct also for the native
guest case: We now simply "commit" what was completed right away, rather
than at the end of a series of preemption/re-start cycles. In fact this
improves overall preemption behavior: There's no longer a potentially
big chunk of work done non-preemptively at the end of the last
"iteration".

Fixes: b2245acc60c3 ("xen/pvshim: memory hotplug")
Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -1461,22 +1461,17 @@ long do_memory_op(unsigned long cmd, XEN
 
         rc = args.nr_done;
 
-        if ( args.preempted )
-            return hypercall_create_continuation(
-                __HYPERVISOR_memory_op, "lh",
-                op | (rc << MEMOP_EXTENT_SHIFT), arg);
-
 #ifdef CONFIG_X86
         if ( pv_shim && op == XENMEM_decrease_reservation )
-            /*
-             * Only call pv_shim_offline_memory when the hypercall has
-             * finished. Note that nr_done is used to cope in case the
-             * hypercall has failed and only part of the extents where
-             * processed.
-             */
-            pv_shim_offline_memory(args.nr_done, args.extent_order);
+            pv_shim_offline_memory(args.nr_done - start_extent,
+                                   args.extent_order);
 #endif
 
+        if ( args.preempted )
+           return hypercall_create_continuation(
+                __HYPERVISOR_memory_op, "lh",
+                op | (rc << MEMOP_EXTENT_SHIFT), arg);
+
         break;
 
     case XENMEM_exchange: