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

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On Wed, Jan 25, 2017 at 12:23 PM, Ricardo Neri
<ricardo.neri-calderon@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:

 - 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.)
 - 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.

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[v3 PATCH 00/10] x86: Enable User-Mode Instruction Prevention Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-01-25 21:30 +0100
  [v3 PATCH 07/10] x86: Add emulation code for UMIP instructions Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-01-25 21:30 +0100
    Re: [v3 PATCH 07/10] x86: Add emulation code for UMIP instructions "H. Peter Anvin" <hpa@zytor.com> - 2017-01-25 21:50 +0100
      Re: [v3 PATCH 07/10] x86: Add emulation code for UMIP instructions Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-01-26 07:00 +0100
  [v3 PATCH 01/10] x86/mpx: Do not use SIB index if index points to R/ESP Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-01-25 21:30 +0100
  [v3 PATCH 09/10] x86: Enable User-Mode Instruction Prevention Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-01-25 21:30 +0100
  [v3 PATCH 03/10] x86/mpx, x86/insn: Relocate insn util functions to a new insn-kernel Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-01-25 21:30 +0100
    Re: [v3 PATCH 03/10] x86/mpx, x86/insn: Relocate insn util  functions to a new insn-kernel Masami Hiramatsu <mhiramat@kernel.org> - 2017-01-26 03:30 +0100
  [v3 PATCH 10/10] selftests/x86: Add tests for User-Mode Instruction Prevention Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-01-25 21:30 +0100
  [v3 PATCH 05/10] x86/insn-kernel: Add support to resolve 16-bit addressing encodings Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-01-25 21:30 +0100
    Re: [v3 PATCH 05/10] x86/insn-kernel: Add support to resolve 16-bit  addressing encodings Andy Lutomirski <luto@amacapital.net> - 2017-01-25 23:00 +0100
      Re: [v3 PATCH 05/10] x86/insn-kernel: Add support to resolve 16-bit addressing encodings "H. Peter Anvin" <hpa@zytor.com> - 2017-01-25 23:10 +0100
      Re: [v3 PATCH 05/10] x86/insn-kernel: Add support to resolve 16-bit  addressing encodings Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-01-26 07:00 +0100
        Re: [v3 PATCH 05/10] x86/insn-kernel: Add support to resolve 16-bit  addressing encodings Andy Lutomirski <luto@amacapital.net> - 2017-01-26 18:20 +0100
          Re: [v3 PATCH 05/10] x86/insn-kernel: Add support to resolve 16-bit  addressing encodings Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-01-27 04:50 +0100
  [v3 PATCH 02/10] x86/mpx: Fail decoding when SIB baseR/EBP is and no displacement is used Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-01-25 21:30 +0100
  [v3 PATCH 08/10] x86/traps: Fixup general protection faults caused by UMIP Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-01-25 21:30 +0100
  [v3 PATCH 04/10] x86/insn-kernel: Add a function to obtain register offset in ModRM Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-01-25 21:30 +0100
    Re: [v3 PATCH 04/10] x86/insn-kernel: Add a function to obtain  register offset in ModRM Masami Hiramatsu <mhiramat@kernel.org> - 2017-01-26 03:20 +0100
      Re: [v3 PATCH 04/10] x86/insn-kernel: Add a function to obtain  register offset in ModRM Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-01-26 07:10 +0100
        Re: [v3 PATCH 04/10] x86/insn-kernel: Add a function to obtain  register offset in ModRM Masami Hiramatsu <mhiramat@kernel.org> - 2017-01-27 09:00 +0100
  Re: [v3 PATCH 00/10] x86: Enable User-Mode Instruction Prevention "H. Peter Anvin" <hpa@zytor.com> - 2017-01-25 21:50 +0100
    Re: [v3 PATCH 00/10] x86: Enable User-Mode Instruction Prevention Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-01-26 07:00 +0100

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