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

[Jan Beulich]

On 10.10.2024 09:51, Hu, Lin1 wrote:
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@suse.com>
>> Sent: Thursday, October 10, 2024 3:10 PM
>> To: Hu, Lin1 <lin1.hu@intel.com>
>> Cc: Binutils <binutils@sourceware.org>; Cui, Lili <lili.cui@intel.com>; H.J. Lu
>> <hjl.tools@gmail.com>
>> Subject: Re: [PATCH 2/2] x86/APX: support JMPABS also in assembler
>>
>> On 10.10.2024 04:41, Hu, Lin1 wrote:
>>> And I think 'movabs moffs64, %rax' and 'movabs imm64, %rax' are two
>> different instructions, they have different opcode.
>>
>> Yes.
>>
>>> According to spec, jmpabs is closer to 'movabs imm64, %rax', because jmpabs
>> only accept imm64.
>>
>> The "because" here is unjustified, at least as long as you can't point me at where
>> this is being said. Hence the "why" on the first part of the sentence remains open.
>>
> 
> On page 233 of Document (https://cdrdv2.intel.com/v1/dl/getContent/784266) it is written "The 64-bit immediate operand is treated an as absolute effective address, which is subject to canonicality checks."
> 
>>
>> All the spec says is that there is a 64-bit field as the only operand. It says nothing
>> on this being an immediate or a displacement. In fact, as
>> (indirectly) pointed out before, the boundary between the two is actually fuzzy in
>> the SDM; the strong distinction between them is something assemblers invented.
> 
> In the document I posted above, I think it's kind of explicitly written clearly.

Then please look at this text taken from the CALL insn page of the SDM:

"The operand can be an immediate value, a general-purpose register, or a
memory location."

Since neither "general-purpose register" nor "memory location" apply to
opcode E8, it must be an immediate. Yet in the assembler we call it a
displacement.

>>> If jmpabs want to accept moffs64, it should use another opcode.
>>> Even if we refer to movabs, we should recognize the possibility of the
>> appearance of another instruction 'jmpabs moffs64'.  Abs suffix is not a
>> constraint.
>>>
>>> And in terms of usage habits, I think people feel 0x123 is a offset rather than a
>> absolute address.  People unfamiliar with jmpabs might see 0x123 and think the
>> address is rip + 0x123 rather then just 0x123.
>>
>> Then they simply missed the point of the "abs" in the mnemonic.
>>
> 
> Maybe, the same thing I think has happened with movabs. I think when people use "movabs moffs64, %rax", they ignore the "abs" in the mnemonic, too. They determine whether it's a relative address

Relative address? It's an absolute address, not a displacement relative to somewhere.

Jan

> or immediate operand by the form of the operand rather than the "abs" in the mnemonic.
> 
> BRs,
> Lin