Re: [v3 PATCH 05/10] x86/insn-kernel: Add support to resolve 16-bit addressing encodings

From: Andy Lutomirski
Date: Wed Jan 25 2017 - 16:59:00 EST

On Wed, Jan 25, 2017 at 12:23 PM, Ricardo Neri
<ricardo.neri-calderon@xxxxxxxxxxxxxxx> wrote:
> Tasks running in virtual-8086 mode will use 16-bit addressing form
> encodings as described in the Intel 64 and IA-32 Architecture Software
> Developer's Manual Volume 2A Section 2.1.5. 16-bit addressing encodings
> differ in several ways from the 32-bit/64-bit addressing form encodings:
> the r/m part of the ModRM byte points to different registers and, in some
> cases, addresses can be indicated by the addition of the value of two
> registers. Also, there is no support for SiB bytes. Thus, a separate
> function is needed to parse this form of addressing.
> Furthermore, virtual-8086 mode tasks will use real-mode addressing. This
> implies that the segment selectors do not point to a segment descriptor
> but are used to compute logical addresses. Hence, there is a need to
> add support to compute addresses using the segment selectors. If segment-
> override prefixes are present in the instructions, they take precedence.
> Lastly, it is important to note that when a tasks is running in virtual-
> 8086 mode and an interrupt/exception occurs, the CPU pushes to the stack
> the segment selectors for ds, es, fs and gs. These are accesible via the
> struct kernel_vm86_regs rather than pt_regs.
> Code for 16-bit addressing encodings is likely to be used only by virtual-
> 8086 mode tasks. Thus, this code is wrapped to be built only if the
> option CONFIG_VM86 is selected.

That's not true. It's used in 16-bit protected mode, too. And there
are (ugh!) six possibilities:

- Normal 32-bit protected mode. This should already work.
- Normal 64-bit protected mode. This should also already work. (I
forget whether a 16-bit SS is either illegal or has no effect in this
- Virtual 8086 mode
- Normal 16-bit protected mode, used by DOSEMU and Wine. (16-bit CS,
16-bit address segment)
- 16-bit CS, 32-bit address segment. IIRC this might be used by some
32-bit DOS programs to call BIOS.
- 32-bit CS, 16-bit address segment. I don't know whether anything uses this.

I don't know if anything you're doing cares about SS's, DS's, etc.
size, but I suspect you'll need to handle 16-bit CS.