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

Ricardo Neri ricardo.neri-calderon at linux.intel.com
Wed Jan 25 23:50:09 CST 2017

On Wed, 2017-01-25 at 13:58 -0800, Andy Lutomirski wrote:
> On Wed, Jan 25, 2017 at 12:23 PM, Ricardo Neri
> <ricardo.neri-calderon at linux.intel.com> 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:

Thanks for the clarification. I will enable the decoding of addresses
for 16-bit as well... and test the emulation code. 
>  - 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
> case.)

For these two cases I am just taking the effective address that the user
space application provides, given that the segment selectors were set
beforehand (and with a base of 0).

copy_to/from_user(kernel_buf, effective_address, sizeof(kernel_buf));

>  - Virtual 8086 mode

In this case I calculate the linear address as:
     (segment_select << 4) + effective address.

>  - 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.

In all these protected modes, are you referring to the size in bits of
the base address of in the descriptor selected in the CS register? In
such a case I would need to get the base address and add it to the
effective address given in the operands of the instructions, right?
> I don't know if anything you're doing cares about SS's, DS's, etc.

For 16-bit addressing encodings, I use DS for the BX, DI and SI
registers, and no register at all. I use SS for BP and SP. This can be
overridden by the segment selector prefixes. For 32/64-bit, I need to
implement segment selector awareness.

> size, but I suspect you'll need to handle 16-bit CS.

Unless I am missing what is special with the 16-bit base address, I only
would need to add that base address to whatever effective address (aka,
offset) is encoded in the ModRM and displacement bytes.

At this moment my implementation is able to decode the ModRM, SiB and
displacement bytes for 16-bit and 32/64-bit address encodings.

Thanks and BR,

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