linux-kernel - RE: [tip:x86/asm] x86/entry/64: Add two more instruction suffixes

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Date:   Tue, 03 Jul 2018 04:06:48 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "David Laight" <David.Laight@...LAB.COM>
Cc:     "Borislav Petkov" <bp@...en8.de>, <brgerst@...il.com>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Andrew Lutomirski" <luto@...nel.org>,
        "Ingo Molnar" <mingo@...nel.org>, <tglx@...utronix.de>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Denys Vlasenko" <dvlasenk@...hat.com>,
        "Josh Poimboeuf" <jpoimboe@...hat.com>,
        <linux-kernel@...r.kernel.org>,
        <linux-tip-commits@...r.kernel.org>, <hpa@...or.com>
Subject: RE: [tip:x86/asm] x86/entry/64: Add two more instruction
 suffixes

>>> On 03.07.18 at 10:46, <David.Laight@...LAB.COM> wrote:
> From: Jan Beulich
>> Sent: 03 July 2018 09:36
> ...
>> As said there, omitting suffixes from instructions in AT&T mode is bad
>> practice when operand size cannot be determined by the assembler from
>> register operands, and is likely going to be warned about by upstream
>> gas in the future (mine does already).
> ...
>> -	bt	$9, EFLAGS(%rsp)		/* interrupts off? */
>> +	btl	$9, EFLAGS(%rsp)		/* interrupts off? */
> 
> Hmmm....
> Does the operand size make any difference at all for the bit instructions?
> I'm pretty sure that the cpus (386 onwards) have always done aligned 32bit
> transfers (the docs never actually said aligned).
> I can't remember whether 64bit mode allows immediates above 31.
> 
> So gas accepting 'btb $n,memory' is giving a false impression of
> what actually happens.

BTB does not exist at all. BTW and (on 64-bit) BTQ do exist though,
and they have behavior differing from BTL. The only AT&T syntax doc
I have says that L is the default suffix to be used, but there are cases
where this wasn't (and maybe still isn't) the case, so omitting a suffix
when register operands aren't available to size instructions has always
been a risky game.

Jan