x86/HVM: limit writes to incoming TSS during task switch

The only field modified (and even that conditionally) is the back link.
Write only that field, and only when it actually has been written to.

Take the opportunity and also ditch the pointless initializer from the
"tss" local variable.

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

--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -2890,7 +2890,7 @@ void hvm_task_switch(
         u32 cr3, eip, eflags, eax, ecx, edx, ebx, esp, ebp, esi, edi;
         u16 es, _3, cs, _4, ss, _5, ds, _6, fs, _7, gs, _8, ldt, _9;
         u16 trace, iomap;
-    } tss = { 0 };
+    } tss;
 
     hvm_get_segment_register(v, x86_seg_gdtr, &gdt);
     hvm_get_segment_register(v, x86_seg_tr, &prev_tr);
@@ -3010,12 +3010,6 @@ void hvm_task_switch(
     regs->esi    = tss.esi;
     regs->edi    = tss.edi;
 
-    if ( (taskswitch_reason == TSW_call_or_int) )
-    {
-        regs->eflags |= X86_EFLAGS_NT;
-        tss.back_link = prev_tr.sel;
-    }
-
     exn_raised = 0;
     if ( hvm_load_segment_selector(x86_seg_es, tss.es, tss.eflags) ||
          hvm_load_segment_selector(x86_seg_cs, tss.cs, tss.eflags) ||
@@ -3025,12 +3019,18 @@ void hvm_task_switch(
          hvm_load_segment_selector(x86_seg_gs, tss.gs, tss.eflags) )
         exn_raised = 1;
 
-    rc = hvm_copy_to_guest_virt(
-        tr.base, &tss, sizeof(tss), PFEC_page_present);
-    if ( rc == HVMCOPY_bad_gva_to_gfn )
-        exn_raised = 1;
-    else if ( rc != HVMCOPY_okay )
-        goto out;
+    if ( taskswitch_reason == TSW_call_or_int )
+    {
+        regs->eflags |= X86_EFLAGS_NT;
+        tss.back_link = prev_tr.sel;
+
+        rc = hvm_copy_to_guest_virt(tr.base + offsetof(typeof(tss), back_link),
+                                    &tss.back_link, sizeof(tss.back_link), 0);
+        if ( rc == HVMCOPY_bad_gva_to_gfn )
+            exn_raised = 1;
+        else if ( rc != HVMCOPY_okay )
+            goto out;
+    }
 
     if ( (tss.trace & 1) && !exn_raised )
         hvm_inject_hw_exception(TRAP_debug, HVM_DELIVER_NO_ERROR_CODE);