The asm was setting regs->cx to zero - use a temp variable to avoid that.
SET_VAR(ES, *(u8 *)(si+0x28+5), 0x93); // access
SET_VAR(ES, *(u16*)(si+0x28+6), 0x0000); // base 31:24/reserved/limit 19:16
+ u16 count = regs->cx;
asm volatile(
// Load new descriptor tables
"lgdtw %%es:0x8(%%si)\n"
// Restore %ds (from %ss)
"movw %%ss, %%ax\n"
"movw %%ax, %%ds\n"
- : "+c"(regs->cx), "+S"(si)
+ : "+c"(count), "+S"(si)
: : "eax", "di"); // XXX - also clobbers %es
set_a20(prev_a20_enable);