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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 21 Nov 2016 10:20:17 -0800
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Subject: Re: What exactly do 32-bit x86 exceptions push on the stack in the CS slot?
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andy Lutomirski <luto@kernel.org>, tedheadster@gmail.com,
        Brian Gerst <brgerst@gmail.com>, George Spelvin <linux@horizon.com>,
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On Mon, Nov 21, 2016 at 7:58 AM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 11/20/16 20:54, hpa@zytor.com wrote:
>>
>> I believe i686+ writes zero, older CPUs leave unchanged.
>
> I should point out that, at least from my memory, the same applies to
> instructions like "movl <seg>".  I can't even remember for sure how the
> behavior differs between "movl <seg>,<reg32>" and "movl <seg>,<mem>";
> I'd have to do some digging.

I have this distinct feeling that there issues with *both* the
register and memory versions.

Because I have this dim memory that on early microarchitectures, even
"mov segment to register" would always only do a 16-bit move, even if
it was encoded as a 32-bit "movl". Although that may be partly because
I know "gas" had some confusion about operand sizes and segment
register instructions, so there might have been toolchain issues too.

I just dug out my old 486 manual on _paper_ (Christ, I still had it):
"Intel486(tm) Microprocessor Family Programmer's Reference Manual".
The "mov" instruction is only documented for r/m16, and it has a
footnote saying "In protected mode, use 16-bit operand size prefix".

I definitely know that the "only write 16 bits" was the case for
memory accesses, but I think it might have been the case even for
register moves. After all, "mov segment register" is actually a
completely different instruction from the normal "mov" instructions,
even if it often shows up together with them in the instruction
descriptions.

The i686 cleaned up a lot of things, but I think this might be an area
where there were differences between i486 and Pentium and all the
clone chips too.

                    Linus