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Date:   Wed, 25 Jan 2017 14:04:30 -0800
From:   "H. Peter Anvin" <>
To:     Andy Lutomirski <>,
        Ricardo Neri <>
CC:     Ingo Molnar <>,
        Thomas Gleixner <>,
        Andy Lutomirski <>,
        Borislav Petkov <>,
        Peter Zijlstra <>,
        Andrew Morton <>,
        Brian Gerst <>,
        Chris Metcalf <>,
        Dave Hansen <>,
        Paolo Bonzini <>,
        Liang Z Li <>,
        Masami Hiramatsu <>,
        Huang Rui <>, Jiri Slaby <>,
        Jonathan Corbet <>,
        "Michael S. Tsirkin" <>,
        Paul Gortmaker <>,
        Vlastimil Babka <>,
        Chen Yucong <>,
        Alexandre Julliard <>,
        Fenghua Yu <>, Stas Sergeev <>,
        "Ravi V. Shankar" <>,
        Shuah Khan <>,
        "" <>,
        X86 ML <>,,,
Subject: Re: [v3 PATCH 05/10] x86/insn-kernel: Add support to resolve 16-bit addressing encodings

Buchbinder <>,Colin Ian King <>,Lorenzo Stoakes <>,Qiaowei Ren <>,Arnaldo Carvalho de Melo <>,Adrian Hunter <>,Kees Cook <>,Thomas Garnier <>,Dmitry Vyukov <>
Message-ID: <>

On January 25, 2017 1:58:27 PM PST, Andy Lutomirski <> wrote:
>On Wed, Jan 25, 2017 at 12:23 PM, Ricardo Neri
><> wrote:
>> Tasks running in virtual-8086 mode will use 16-bit addressing form
>> encodings as described in the Intel 64 and IA-32 Architecture
>> Developer's Manual Volume 2A Section 2.1.5. 16-bit addressing
>> differ in several ways from the 32-bit/64-bit addressing form
>> the r/m part of the ModRM byte points to different registers and, in
>> 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.
>> implies that the segment selectors do not point to a segment
>> but are used to compute logical addresses. Hence, there is a need to
>> add support to compute addresses using the segment selectors. If
>> override prefixes are present in the instructions, they take
>> Lastly, it is important to note that when a tasks is running in
>> 8086 mode and an interrupt/exception occurs, the CPU pushes to the
>> the segment selectors for ds, es, fs and gs. These are accesible via
>> struct kernel_vm86_regs rather than pt_regs.
>> Code for 16-bit addressing encodings is likely to be used only by
>> 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.

Only the CS bitness matters for the purpose of addressing modes; the SS bitness (which has no effect in 64-bit mode) only matters for implicit stack references unless I'm completely out to sea.
Sent from my Android device with K-9 Mail. Please excuse my brevity.

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