linux-kernel - Re: [RFC][PATCH] x86: proposed new ARCH_CAPABILITIES MSR bit for RSB-underflow

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Message-ID: <CA+55aFyg-uS_hE6f2t5gOrwaCMXErPvGA7tjfhmXZmZOPzi8Ww@mail.gmail.com>
Date:   Fri, 16 Feb 2018 11:43:23 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        Rik van Riel <riel@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Kosina <jikos@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Kees Cook <keescook@...gle.com>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Paul Turner <pjt@...gle.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Mallick, Asit K" <asit.k.mallick@...el.com>
Subject: Re: [RFC][PATCH] x86: proposed new ARCH_CAPABILITIES MSR bit for RSB-underflow

On Fri, Feb 16, 2018 at 11:38 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Of course, your patch still doesn't allow for "we claim to be skylake
> for various other independent reasons, but the RSB issue is fixed".

.. maybe nobody ever has a reason to do that, though?

Who knows, virtualization people may simply want the user to specify
the model, but then make the Spectre decisions be based on actual
hardware capabilities (whether those are "current" or "some minimum
base").

Two bits allow that. One bit means "if you claim you're running
skylake, we'll always have to stuff, whether you _really_ are or not".

                   Linus