fix HVM hypercall preemption causing guest crash (changeset: 16925)

Signed-off-by: Yu Zhao <yu.zhao@intel.com>

--- old/xen/arch/x86/domain.c   2008-01-29 12:45:46.000000000 +0800
+++ new/xen/arch/x86/domain.c   2008-01-29 13:34:31.819185813 +0800
@@ -1506,7 +1506,12 @@ unsigned long hypercall_create_continuat
     {
         regs       = guest_cpu_user_regs();
         regs->eax  = op;
-        regs->eip -= 2;  /* re-execute 'syscall' / 'int 0x82' */
+        /*
+         * For PV guest, we update EIP to re-execute 'syscall' / 'int 0x82';
+         * HVM does not need this since 'vmcall' / 'vmmcall' is fault-like.
+         */
+        if ( !is_hvm_vcpu(current) )
+            regs->eip -= 2;

 #ifdef __x86_64__
         if ( !is_hvm_vcpu(current) ?