linux-kernel - Re: [PATCH 03/16] x86/entry/32: Leave the kernel via the trampoline stack

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Message-ID: <20180117141006.GR28161@8bytes.org>
Date:   Wed, 17 Jan 2018 15:10:06 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Brian Gerst <brgerst@...il.com>
Cc:     Andy Lutomirski <luto@...nel.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>,
        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 03/16] x86/entry/32: Leave the kernel via the trampoline
 stack

On Wed, Jan 17, 2018 at 05:57:53AM -0800, Brian Gerst wrote:
> On Wed, Jan 17, 2018 at 1:24 AM, Joerg Roedel <joro@...tes.org> wrote:

> > I have no real idea on how to switch back to the entry stack without
> > access to per_cpu variables. I also can't access the cpu_entry_area for
> > the cpu yet, because for that we need to be on the entry stack already.
> 
> Switch to the trampoline stack before loading user segments.

That requires to copy most of pt_regs from task- to trampoline-stack,
not sure if that is faster than temporily restoring kernel %fs.


	Joerg