linux-kernel - Re: [PATCH 00/31 v2] PTI support for x86

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Message-ID: <a5e37af1-cfa3-1284-cb1c-3912c30505e3@citrix.com>
Date:   Fri, 9 Feb 2018 21:28:08 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Pavel Machek <pavel@....cz>, Andy Lutomirski <luto@...nel.org>
Cc:     Joerg Roedel <joro@...tes.org>,
        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>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, 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>, Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 00/31 v2] PTI support for x86_32

On 09/02/2018 21:09, Pavel Machek wrote:
> On Fri 2018-02-09 17:47:43, Andy Lutomirski wrote:
>> PCID bit set in CPUID should print a big fat warning like "WARNING:
>> you are using 32-bit PTI on a 64-bit PCID-capable CPU.  Your
>> performance will increase dramatically if you switch to a 64-bit
>> kernel."
> Hardware supports PCID even on 32-bit kernels, no?

Attempting to set CR4.PCIDE is disallowed outside of long mode.  It is
strictly a 64bit-only feature.

~Andrew