[prev in list] [next in list] [prev in thread] [next in thread] 

List:       wine-devel
Subject:    Re: [v6 PATCH 03/21] x86/mpx: Do not use R/EBP as base in the SIB byte with Mod = 0
From:       Ricardo Neri <ricardo.neri-calderon () linux ! intel ! com>
Date:       2017-04-27 22:49:58
Message-ID: 1493333398.74182.10.camel () ranerica-desktop
[Download RAW message or body]

On Wed, 2017-04-26 at 10:05 +0200, Borislav Petkov wrote:
> On Tue, Apr 25, 2017 at 07:04:20PM -0700, Ricardo Neri wrote:
> > For the specific case of ModRM.mod being 0, I feel I need to clarify
> > that REX.B is not decoded and if SIB.base is %r13 the base is also 0.
> 
> Well, that all doesn't matter. The rule is this:
> 
> ModRM.mod == 00b and ModRM.r/m == 101b -> effective address: disp32
> 
> See Table 2-2. "32-Bit Addressing Forms with the ModR/M Byte" in the SDM.

You are right. This summarizes the rule. Then I will shorten the
comment.
> 
> So the base register is not used. How that base register is specified
> then doesn't matter (undecoded REX bits or not).
> 
> > This comment adds clarity because REX.X is decoded when determining
> > SIB.index.
> 
> Well, that's a different thing. The REX bits participating in the SIB
> fields don't matter about this particular case. We only want to say that
> we're returning a disp32 without a base register and the comment should
> keep it simple without extraneous information.
> 
> I know, you want to mention what Table 2-5. "Special Cases of REX
> Encodings" says but we should avoid unnecessary content in the comment.
> People who want details can stare at the manuals - the comment should
> only document what that particular case is.
> 
> Btw, you could write it even better:
> 
> 	if (!X86_MODRM_MOD(insn->modrm.value) && X86_MODRM_RM(insn->modrm.value) == 5)
> 
> and then it is basically a 1:1 copy of the rule from Table 2-2.

It is!

Thanks and BR,
Ricardo




[prev in list] [next in list] [prev in thread] [next in thread]