[PATCH] x86/APX: limit printing of REX2 payload

[Jan Beulich]

On 09.09.2026 08:36, H.J. Lu wrote:
> On Wed, Sep 9, 2026 at 1:52 PM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 09.09.2026 07:50, Jiang, Haochen wrote:
>>>> From: Jan Beulich <jbeulich@suse.com>
>>>> Sent: Tuesday, September 8, 2026 11:11 PM
>>>>
>>>> When set bits were properly consumed (and hence all we want is an
>>>> indication that REX2-encoding is in use), print merely {rex2}, thus also
>>>> better matching original input.
>>>>
>>>> While effectively copying parts of an existing conditional, also simplify
>>>> the original: There's no need to apply the 3-bit mask twice.
>>>>
>>>> While adjusting testcases, also escape curly braces.
>>>
>>> Actually, when I am working on BHI_CTRL patch, I have the same question
>>> for why we are printing {rex2 0x88}. Thus, I asked Lili for that. Lili told me
>>> that back to the very beginning, it is H.J requested to add those additional
>>> info to express the rex2 prefix.
>>
>> And we still do when there are unconsumed bits.
>>
>>> Maybe we need H.J's input on this.
>>
>> Sure, I'm not going to commit right away.
>>
> 
> {rex} and {rex2} are different.
> 
> [hjl@gnu-tgl-3 tmp]$ cat x.s
> {rex} movl %eax,%ebx
> {rex2} movl %eax,%ebx
> [hjl@gnu-tgl-3 tmp]$ gcc -c x.s
> [hjl@gnu-tgl-3 tmp]$ objdump -dw x.o
> 
> x.o:     file format elf64-x86-64
> 
> 
> Disassembly of section .text:
> 
> 0000000000000000 <.text>:
>    0: 40 89 c3             rex mov %eax,%ebx
>    3: d5 00 89 c3           {rex2 0x0} mov %eax,%ebx

What useful extra information does the 0x0 supply here? Same as in
the IBHF example, where the 0x88 is entirely redundant. In fact
printing bit 7 is _always_ redundant (unless the prefix was printed
as a "standalone" one), as it necessarily participated in decode.

> [hjl@gnu-tgl-3 tmp]$
> 
> {rex} maps to 0x40.  But {rex2} maps to different 2 bytes.
> {rex2 0x0} tells what these 2 bytes are.

Whether it's one or two bytes isn't relevant. "rex" alone also doesn't
make things entirely unambiguous, unless you know that with any of the
low 4 bits set it wouldn't be plain "rex". IOW the rex printing only
indicates (consumed and unconsumed) bits which are _set_ (and only if
there are unconsumed bits), plus the presence of a (redundant) REX
prefix. rex2 printing otoh indicates all bits (by printing the payload
byte as-is, instead of deciphering the bits).

>   I prefer to keep it ASIS.

Please can we avoid (a) unnecessary clutter and (b) the inconsistency.

Plus, as said in the patch description, {rex2} is what the assembler
accepts. I recall you stressing that disassembler output would,
wherever possible, better match what the assembler accepts as input.

Jan