linux-kernel - Re: [patch V2 1/5] x86/kaiser: Respect disabled CPU features

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Message-ID: <alpine.DEB.2.20.1711271416410.1799@nanos>
Date:   Mon, 27 Nov 2017 14:18:44 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Brian Gerst <brgerst@...il.com>
cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>, Linux-MM <linux-mm@...ck.org>,
        michael.schwarz@...k.tugraz.at, moritz.lipp@...k.tugraz.at,
        richard.fellner@...dent.tugraz.at
Subject: Re: [patch V2 1/5] x86/kaiser: Respect disabled CPU features

On Mon, 27 Nov 2017, Brian Gerst wrote:
> On Mon, Nov 27, 2017 at 6:47 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > On Mon, 27 Nov 2017, Peter Zijlstra wrote:
> >> On Mon, Nov 27, 2017 at 12:14:04AM +0100, Thomas Gleixner wrote:
> >> > PAGE_NX and PAGE_GLOBAL might be not supported or disabled on the command
> >> > line, but KAISER sets them unconditionally.
> >>
> >> So KAISER is x86_64 only, right? AFAIK there is no x86_64 without NX
> >> support. So would it not make sense to mandate NX for KAISER?, that is
> >> instead of making "noexec" + KAISER work, make "noexec" kill KAISER +
> >> emit a warning.
> >
> > OTOH, disabling NX is a simple way to verify that DEBUG_WX works correctly
> > also on the shadow maps.
> >
> > But surely we can drop the PAGE_GLOBAL thing, as all 64bit systems have it.
> 
> I seem to recall that some virtualized environments (maybe Xen?) don't
> support global pages.

Uuurg. Ok, we leave it as is for now. Better safe than sorry. It does no
harm.

Thanks,

	tglx