linux-kernel - Re: [patch 13/16] x86/ldt: Introduce LDT write fault handler

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Date:   Tue, 12 Dec 2017 21:37:52 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Hansen <dave.hansen@...el.com>
cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andy Lutomirsky <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bpetkov@...e.de>,
        Greg KH <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Will Deacon <will.deacon@....com>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [patch 13/16] x86/ldt: Introduce LDT write fault handler

On Tue, 12 Dec 2017, Dave Hansen wrote:

> On 12/12/2017 11:21 AM, Thomas Gleixner wrote:
> > The only critical interaction is the return to user path (user CS/SS) and
> > we made sure with the LAR touching that these are precached in the CPU
> > before we go into fragile exit code.
> 
> How do we make sure that it _stays_ cached?
> 
> Surely there is weird stuff like WBINVD or SMI's that can come at very
> inconvenient times and wipe it out of the cache.

This does not look like cache in the sense of memory cache. It seems to be
CPU internal state and I just stuffed WBINVD and alternatively CLFLUSH'ed
the entries after the 'touch' via LAR. Still works.

Thanks,

	tglx