x86/HVM: use single (atomic) MOV for aligned emulated writes

Using memcpy() may result in multiple individual byte accesses
(dependening how memcpy() is implemented and how the resulting insns,
e.g. REP MOVSB, get carried out in hardware), which isn't what we
want/need for carrying out guest insns as correctly as possible. Fall
back to memcpy() only for accesses not 2, 4, or 8 bytes in size.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/hvm/emulate.c

@@ -1342,7 +1342,14 @@ static int hvmemul_write(
     if ( !mapping )
         return linear_write(addr, bytes, p_data, pfec, hvmemul_ctxt);
 
-    memcpy(mapping, p_data, bytes);
+    /* Where possible use single (and hence generally atomic) MOV insns. */
+    switch ( bytes )
+    {
+    case 2: write_u16_atomic(mapping, *(uint16_t *)p_data); break;
+    case 4: write_u32_atomic(mapping, *(uint32_t *)p_data); break;
+    case 8: write_u64_atomic(mapping, *(uint64_t *)p_data); break;
+    default: memcpy(mapping, p_data, bytes);                break;
+    }
 
     hvmemul_unmap_linear_addr(mapping, addr, bytes, hvmemul_ctxt);