linux-kernel - Re: [PATCH 38/39] x86/mm/pti: Add Warning when booting on a PCID capable CPU

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Date:   Fri, 13 Jul 2018 11:59:44 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>,
        "David H . Gutteridge" <dhgutteridge@...patico.ca>,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 38/39] x86/mm/pti: Add Warning when booting on a PCID
 capable CPU

On Wed, Jul 11, 2018 at 4:29 AM, Joerg Roedel <joro@...tes.org> wrote:
> From: Joerg Roedel <jroedel@...e.de>
>
> Warn the user in case the performance can be significantly
> improved by switching to a 64-bit kernel.

...

> +#ifdef CONFIG_X86_32
> +       if (boot_cpu_has(X86_FEATURE_PCID)) {

I'm a bit confused. Wouldn't the setup_clear_cpu_cap() call in
early_identify_cpu() prevent this from working?

Boris, do we have a straightforward way to ask "does the CPU advertise
this feature in CPUID regardless of whether we have it enabled right
now"?

--Andy