diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index 14c5186fe7..6eec5dc86d 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -3721,6 +3721,9 @@ static void x86_cpu_common_class_init(ObjectClass *oc, void *data)
     cc->write_elf32_qemunote = x86_cpu_write_elf32_qemunote;
     cc->vmsd = &vmstate_x86_cpu;
 #endif
+    /* CPU_NB_REGS * 2 = general regs + xmm regs
+     * 25 = eip, eflags, 6 seg regs, st[0-7], fctrl,...,fop, mxcsr.
+     */
     cc->gdb_num_core_regs = CPU_NB_REGS * 2 + 25;
 #ifndef CONFIG_USER_ONLY
     cc->debug_excp_handler = breakpoint_handler;