[PATCH 2/2] x86/APX: support JMPABS also in assembler

[Jan Beulich]

On 11.10.2024 04:51, Cui, Lili wrote:
>> On 10.10.2024 10:57, Cui, Lili wrote:
>>>> -----Original Message-----
>>>> From: Jan Beulich <jbeulich@suse.com>
>>>> Sent: Thursday, October 10, 2024 3:24 PM
>>>>
>>>> On 10.10.2024 04:54, Cui, Lili wrote:
>>>>> 'jmpabs $foo' and 'jmpabs foo' are really two different things, if
>>>>> used as an
>>>> offset, its access range is different from that of an immediate value.
>>>>
>>>> Can you clarify what you mean here, please? What's "access range"?
>>>>
>>>
>>> Forget it, I thought wrong before.
>>>
>>>>> supporting both formats will confuse users.
>>>>
>>>> That's your view. Mine is that already the prior discussion here has
>>>> proven that it'll actually help users, by allowing them freedom in
>>>> how to write their code.
>>>>
>>>>> Is this an immediate value or an offset? But in fact, this is an
>>>>> immediate
>>>> value.
>>>>
>>>> No more or less than the immediates in e.g. opcodes E8 and E9. As
>>>> said in a reply to Lin, the distinction between "immediate" and
>> "displacement"
>>>> is blurred anyway in the SDM.
>>>
>>> I checked the documentation for E8 and E9, and they are both offsets, so there
>> is no ambiguity.
>>>
>>> E8: Call near, relative, displacement relative to next instruction.
>>> E9: Jump near, relative, displacement relative to next instruction. Not
>> supported in 64-bit mode.
> E9 : JMP rel32 
>>
>> Quoting the same piece from the CALL page of the SDM for you then:
>> "The operand can be an immediate value, a general-purpose register, or a
>> memory location." Note the word "immediate" and how it's not an immediate in
>> the assembler.
>>
> 
> Those who are familiar with this instruction should know that it is an offset. If some information descriptions are unclear or inaccurate, please read it together with all the relevant information.

Thing is (and I said this before) - what the manual says, which is focused
on insn encoding - isn't always tightly connected to how something wants /
needs expressing in assembly source. What is being said there can be taken
as guideline, but needs to further be taken with a grain of salt.

> Even if the description here is unclear, we should not mix immediate and offset together, which will make things more and more confusing and cause trouble for those who have a clear concept. For those who are clear, the first reaction is that the linker will recalculate and backfill the absolute address based on the current offset, but that's not your intention.

I'm afraid I can't put the pieces here together to give a consistent picture.
Yes, the linker _will_ recalculate and fill absolute addresses. That's true
for all of

	movabs	$symbol, %rax
	movabs	symbol, %rax
	jmpabs	$symbol, %rax
	jmpabs	symbol, %rax

Jan