x86: make hypercall preemption checks consistent

- never preempt on the first iteration (ensure forward progress)
- never preempt on the last iteration (pointless/wasteful)

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

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -2934,7 +2934,7 @@ long do_mmuext_op(
 
     for ( i = 0; i < count; i++ )
     {
-        if ( curr->arch.old_guest_table || hypercall_preempt_check() )
+        if ( curr->arch.old_guest_table || (i && hypercall_preempt_check()) )
         {
             rc = -EAGAIN;
             break;
@@ -3481,7 +3481,7 @@ long do_mmu_update(
 
     for ( i = 0; i < count; i++ )
     {
-        if ( curr->arch.old_guest_table || hypercall_preempt_check() )
+        if ( curr->arch.old_guest_table || (i && hypercall_preempt_check()) )
         {
             rc = -EAGAIN;
             break;
--- a/xen/arch/x86/mm/hap/hap.c
+++ b/xen/arch/x86/mm/hap/hap.c
@@ -326,7 +326,7 @@ hap_set_allocation(struct domain *d, uns
     else
         pages -= d->arch.paging.hap.p2m_pages;
 
-    while ( d->arch.paging.hap.total_pages != pages )
+    for ( ; ; )
     {
         if ( d->arch.paging.hap.total_pages < pages )
         {
@@ -355,6 +355,8 @@ hap_set_allocation(struct domain *d, uns
             d->arch.paging.hap.total_pages--;
             free_domheap_page(pg);
         }
+        else
+            break;
 
         /* Check to see if we need to yield and try again */
         if ( preempted && hypercall_preempt_check() )
--- a/xen/arch/x86/mm/p2m-pod.c
+++ b/xen/arch/x86/mm/p2m-pod.c
@@ -242,7 +242,8 @@ p2m_pod_set_cache_target(struct p2m_doma
 
         p2m_pod_cache_add(p2m, page, order);
 
-        if ( hypercall_preempt_check() && preemptible )
+        if ( preemptible && pod_target != p2m->pod.count &&
+             hypercall_preempt_check() )
         {
             ret = -EAGAIN;
             goto out;
@@ -286,7 +287,8 @@ p2m_pod_set_cache_target(struct p2m_doma
 
             put_page(page+i);
 
-            if ( hypercall_preempt_check() && preemptible )
+            if ( preemptible && pod_target != p2m->pod.count &&
+                 hypercall_preempt_check() )
             {
                 ret = -EAGAIN;
                 goto out;
--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -1672,7 +1672,7 @@ static unsigned int sh_set_allocation(st
     SHADOW_PRINTK("current %i target %i\n", 
                    d->arch.paging.shadow.total_pages, pages);
 
-    while ( d->arch.paging.shadow.total_pages != pages ) 
+    for ( ; ; )
     {
         if ( d->arch.paging.shadow.total_pages < pages ) 
         {
@@ -1707,6 +1707,8 @@ static unsigned int sh_set_allocation(st
             d->arch.paging.shadow.total_pages--;
             free_domheap_page(sp);
         }
+        else
+            break;
 
         /* Check to see if we need to yield and try again */
         if ( preempted && hypercall_preempt_check() )
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -3596,13 +3596,6 @@ long do_set_trap_table(XEN_GUEST_HANDLE_
 
     for ( ; ; )
     {
-        if ( hypercall_preempt_check() )
-        {
-            rc = hypercall_create_continuation(
-                __HYPERVISOR_set_trap_table, "h", traps);
-            break;
-        }
-
         if ( copy_from_guest(&cur, traps, 1) )
         {
             rc = -EFAULT;
@@ -3623,6 +3616,13 @@ long do_set_trap_table(XEN_GUEST_HANDLE_
             init_int80_direct_trap(curr);
 
         guest_handle_add_offset(traps, 1);
+
+        if ( hypercall_preempt_check() )
+        {
+            rc = hypercall_create_continuation(
+                __HYPERVISOR_set_trap_table, "h", traps);
+            break;
+        }
     }
 
     return rc;
--- a/xen/arch/x86/x86_64/compat/traps.c
+++ b/xen/arch/x86/x86_64/compat/traps.c
@@ -329,13 +329,6 @@ int compat_set_trap_table(XEN_GUEST_HAND
 
     for ( ; ; )
     {
-        if ( hypercall_preempt_check() )
-        {
-            rc = hypercall_create_continuation(
-                __HYPERVISOR_set_trap_table, "h", traps);
-            break;
-        }
-
         if ( copy_from_guest(&cur, traps, 1) )
         {
             rc = -EFAULT;
@@ -353,6 +346,13 @@ int compat_set_trap_table(XEN_GUEST_HAND
             init_int80_direct_trap(current);
 
         guest_handle_add_offset(traps, 1);
+
+        if ( hypercall_preempt_check() )
+        {
+            rc = hypercall_create_continuation(
+                __HYPERVISOR_set_trap_table, "h", traps);
+            break;
+        }
     }
 
     return rc;