Date: Wed, 26 Apr 2017 10:05:42 +0200
From: Borislav Petkov <bp@suse.de>
To: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Brian Gerst <brgerst@gmail.com>, Chris Metcalf <cmetcalf@mellanox.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Masami Hiramatsu <mhiramat@kernel.org>, Huang Rui <ray.huang@amd.com>,
        Jiri Slaby <jslaby@suse.cz>, Jonathan Corbet <corbet@lwn.net>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Vlastimil Babka <vbabka@suse.cz>, Chen Yucong <slaoub@gmail.com>,
        Alexandre Julliard <julliard@winehq.org>, Stas Sergeev <stsp@list.ru>,
        Fenghua Yu <fenghua.yu@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Shuah Khan <shuah@kernel.org>, linux-kernel@vger.kernel.org,
        x86@kernel.org, linux-msdos@vger.kernel.org, wine-devel@winehq.org,
        Adam Buchbinder <adam.buchbinder@gmail.com>,
        Colin Ian King <colin.king@canonical.com>,
        Lorenzo Stoakes <lstoakes@gmail.com>,
        Qiaowei Ren <qiaowei.ren@intel.com>,
        Nathan Howard <liverlint@gmail.com>,
        Adan Hawthorn <adanhawthorn@gmail.com>, Joe Perches <joe@perches.com>
Subject: Re: [v6 PATCH 03/21] x86/mpx: Do not use R/EBP as base in the SIB
 byte with Mod = 0
Message-ID: <20170426080542.pannewbvzeqn5kj6@pd.tnic>
References: <20170308003254.27833-1-ricardo.neri-calderon@linux.intel.com>
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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.

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.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--