linux-kernel - Re: [PATCH] x86/retpoline: Fill RSB on context switch for affected CPUs

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Date:   Fri, 12 Jan 2018 10:02:24 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     David Woodhouse <dwmw@...zon.co.uk>
Cc:     Paul Turner <pjt@...gle.com>, LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, tglx@...utronix.de,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk,
        x86@...nel.org, thomas.lendacky@....com,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH] x86/retpoline: Fill RSB on context switch for affected
 CPUs

> +	if ((!boot_cpu_has(X86_FEATURE_PTI) &&
> +	     !boot_cpu_has(X86_FEATURE_SMEP)) || is_skylake_era()) {
> +		setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
> +		pr_info("Filling RSB on context switch\n");

We need to do more things for Skylake (like idle and interrupt fill
and possibly deep call cahin), so I don't think it makes sense to

- have an individual flag for each of these. It can be just a single
flag that enables all of this for Skylake

- print something for each of them. that will just be very noisy
without any useful benefit to the user.

-Andi