Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor

Madhavan T. Venkataraman
2020-08-03T17:00:07Z

On 8/3/20 11:57 AM, David Laight wrote:
> From: Madhavan T. Venkataraman
>> Sent: 03 August 2020 17:03
>>
>> On 8/3/20 3:27 AM, David Laight wrote:
>>> From: Mark Rutland
>>>> Sent: 31 July 2020 19:32
>>> ...
>>>>> It requires PC-relative data references. I have not worked on all architectures.
>>>>> So, I need to study this. But do all ISAs support PC-relative data references?
>>>> Not all do, but pretty much any recent ISA will as it's a practical
>>>> necessity for fast position-independent code.
>>> i386 has neither PC-relative addressing nor moves from %pc.
>>> The cpu architecture knows that the sequence:
>>> call 1f
>>> 1: pop %reg
>>> is used to get the %pc value so is treated specially so that
>>> it doesn't 'trash' the return stack.
>>>
>>> So PIC code isn't too bad, but you have to use the correct
>>> sequence.
>> Is that true only for 32-bit systems only? I thought RIP-relative addressing was
>> introduced in 64-bit mode. Please confirm.
> I said i386 not amd64 or x86-64.

I am sorry. My bad.

>
> So yes, 64bit code has PC-relative addressing.
> But I'm pretty sure it has no other way to get the PC itself
> except using call - certainly nothing in the 'usual' instructions.

OK.

Madhavan