common: make hypercall preemption checks consistent

- never preempt on the first iteration (ensure forward progress)
- do cheap checks first

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -63,7 +63,7 @@ static void increase_reservation(struct 
 
     for ( i = a->nr_done; i < a->nr_extents; i++ )
     {
-        if ( hypercall_preempt_check() )
+        if ( i != a->nr_done && hypercall_preempt_check() )
         {
             a->preempted = 1;
             goto out;
@@ -109,7 +109,7 @@ static void populate_physmap(struct memo
 
     for ( i = a->nr_done; i < a->nr_extents; i++ )
     {
-        if ( hypercall_preempt_check() )
+        if ( i != a->nr_done && hypercall_preempt_check() )
         {
             a->preempted = 1;
             goto out;
@@ -268,7 +268,7 @@ static void decrease_reservation(struct 
 
     for ( i = a->nr_done; i < a->nr_extents; i++ )
     {
-        if ( hypercall_preempt_check() && i != a->nr_done )
+        if ( i != a->nr_done && hypercall_preempt_check() )
         {
             a->preempted = 1;
             goto out;
@@ -398,7 +398,8 @@ static long memory_exchange(XEN_GUEST_HA
           i < (exch.in.nr_extents >> in_chunk_order);
           i++ )
     {
-        if ( hypercall_preempt_check() )
+        if ( i != (exch.nr_exchanged >> in_chunk_order) &&
+             hypercall_preempt_check() )
         {
             exch.nr_exchanged = i << in_chunk_order;
             rcu_unlock_domain(d);
--- a/xen/common/multicall.c
+++ b/xen/common/multicall.c
@@ -52,7 +52,7 @@ do_multicall(
 
     for ( i = 0; !rc && i < nr_calls; i++ )
     {
-        if ( hypercall_preempt_check() )
+        if ( i && hypercall_preempt_check() )
             goto preempted;
 
         if ( unlikely(__copy_from_guest(&mcs->call, call_list, 1)) )
--- a/xen/drivers/char/console.c
+++ b/xen/drivers/char/console.c
@@ -375,12 +375,12 @@ static DECLARE_SOFTIRQ_TASKLET(notify_do
 static long guest_console_write(XEN_GUEST_HANDLE_PARAM(char) buffer, int count)
 {
     char kbuf[128];
-    int kcount;
+    int kcount = 0;
     struct domain *cd = current->domain;
 
     while ( count > 0 )
     {
-        if ( hypercall_preempt_check() )
+        if ( kcount && hypercall_preempt_check() )
             return hypercall_create_continuation(
                 __HYPERVISOR_console_io, "iih",
                 CONSOLEIO_write, count, buffer);