> > Something like this:
> > ENTRY(debug)
> > 	testl $0x3, 4(%esp)
> > 	jz handle_kernel_mode_debug
> > 
> 
> This check is insufficient, it can go the wrong way if the interrupted
> taks is in vm 86 mode (and open a big security hole, I believe).
Right.
This test from ret_from_intr could be used instead:

push %es
push %ds
push %eax
movl 8(%esp), %eax
movb 4(%esp), %al
testl $(VM_MASK | 3), %eax
jz handle_kernel_mode

There is however a potential register stall for the eax read after the
al write.

BTW, vm86 has the VM flag set so the comment in ret_from_intr is wrong.
The code seems to be right though.