linux-kernel - Re: [Patch v4 03/18] x86/speculation: Reorganize cpu_show

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Date:   Sat, 3 Nov 2018 19:07:36 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
cc:     Jiri Kosina <jikos@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [Patch v4 03/18] x86/speculation: Reorganize cpu_show_common()

Tim,

On Tue, 30 Oct 2018, Tim Chen wrote:
> Extract the logic to show IBPB, STIBP usages in cpu_show_common()
> into helper functions.
> 
> Later patches will add other userspace Spectre v2 mitigation modes.
> This patch makes it easy to show IBPB and STIBP
> usage scenario according to the mitigation mode.

First of all, I asked you before to do:

# git grep 'This patch' Documentation/process

This leads you to:

 "Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
  instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
  to do frotz", as if you are giving orders to the codebase to change
  its behaviour."

Documentation is there for a reason.

Aside of that, I'd really have a hard time to figure out what you are
trying to say, if I didn't have the context already. Change logs need to
make sense on their own. So something like this:

  The Spectre V2 printout in cpu_show_common() handles conditionals for the
  various mitigation methods directly in the sprintf() argument list. That's
  hard to read and will become unreadable if more complex decisions need to
  be made for a particular method.

  Move the conditionals for STIBP and IBPB string selection into helper
  functions, so they can be extended later on.

follows the obvious ordering for change logs:

  1) Describe context and problem

  2) Describe the solution

and is understandable without needing to know about the context in which
this change was developed.

Hmm? This is a suggestion, feel free to rewrite it in you own words. The
same applies to other change logs as well. I won't comment on those.

>  static ssize_t cpu_show_common(struct device *dev, struct device_attribute *attr,
>  			       char *buf, unsigned int bug)
>  {
> @@ -872,9 +888,8 @@ static ssize_t cpu_show_common(struct device *dev, struct device_attribute *attr
>  
>  	case X86_BUG_SPECTRE_V2:
>  		return sprintf(buf, "%s%s%s%s%s%s\n", spectre_v2_strings[spectre_v2_enabled],
> -			       boot_cpu_has(X86_FEATURE_USE_IBPB) ? ", IBPB" : "",
>  			       boot_cpu_has(X86_FEATURE_USE_IBRS_FW) ? ", IBRS_FW" : "",
> -			       (x86_spec_ctrl_base & SPEC_CTRL_STIBP) ? ", STIBP" : "",
> +			       ibpb_state(), stibp_state(),
>  			       boot_cpu_has(X86_FEATURE_RSB_CTXSW) ? ", RSB filling" : "",
>  			       spectre_v2_module_string());

Any particular reason for changing the output ordering here? If yes, then
the changelog should mention it. If no, why?

Thanks,

	tglx