linux-kernel - RE: [PATCH v2] x86: modernize sync

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Date:   Wed, 21 Nov 2018 07:41:41 -0700
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "David Laight" <David.Laight@...LAB.COM>
Cc:     <mingo@...e.hu>, <tglx@...utronix.de>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
        "Juergen Gross" <jgross@...e.com>, <linux-kernel@...r.kernel.org>,
        <hpa@...or.com>
Subject: RE: [PATCH v2] x86: modernize sync_bitops.h

>>> On 21.11.18 at 14:49, <David.Laight@...LAB.COM> wrote:
> From: Jan Beulich
>> Sent: 21 November 2018 13:03
>> 
>> >>> On 21.11.18 at 12:55, <David.Laight@...LAB.COM> wrote:
>> > From: Jan Beulich
>> >> Sent: 21 November 2018 10:11
>> >>
>> >> Add missing insn suffixes and use rmwcc.h just like was (more or less)
>> >> recently done for bitops.h as well.
>> >
>> > Why? bts (etc) on memory don't really have an 'operand size'.
>> 
>> Of course they do - depending on operand size they operate on
>> 2-, 4-, or 8-byte quantities. When the second operand is a
>> register, the suffix is redundant (but doesn't hurt), but when
>> the second operand is an immediate, the assembler (in AT&T
>> syntax) has no way of knowing what operand size you mean.
> 
> You need to RTFM.

Excuse me? How about you look at this table from the SDM
(format of course comes out better in the .pdf):

0F AB /r BTS r/m16, r16 MR Valid Valid Store selected bit in CF flag and set.
0F AB /r BTS r/m32, r32 MR Valid Valid Store selected bit in CF flag and set.
REX.W + 0F AB /r BTS r/m64, r64 MR Valid N.E. Store selected bit in CF flag and set.
0F BA /5 ib BTS r/m16, imm8 MI Valid Valid Store selected bit in CF flag and set.
0F BA /5 ib BTS r/m32, imm8 MI Valid Valid Store selected bit in CF flag and set.
REX.W + 0F BA /5 ib BTS r/m64, imm8 MI Valid N.E. Store selected bit in CF flag and set.

Please read manuals yourself before making such statements.

> Regardless of the 'operand size' the 'bit' instructions do a 32 bit aligned
> 32 bit wide read/modify/write cycle.
> 
> The 'operand size' does affect whether the bit number (which is signed)
> comes from %cl (8 bits), %cx (16 bits), %rcx (32 bits) or (%ecx) 64 bits.
> But that is implicit in the register name used.

There is no form with %cl as operand. Instead there are forms with
an immediate operand. 

Jan