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Date: Wed, 17 Jan 2018 15:10:06 +0100
From: Joerg Roedel <joro@8bytes.org>
Subject: Re: [PATCH 03/16] x86/entry/32: Leave the kernel via the trampoline
 stack
Message-ID: <20180117141006.GR28161@8bytes.org>
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To: Brian Gerst <brgerst@gmail.com>
Cc: Andy Lutomirski <luto@kernel.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>, "H . Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>, Linus Torvalds <torvalds@linux-foundation.org>, Dave Hansen <dave.hansen@intel.com>, Josh Poimboeuf <jpoimboe@redhat.com>, Juergen Gross <jgross@suse.com>, Peter Zijlstra <peterz@infradead.org>, Borislav Petkov <bp@alien8.de>, Jiri Kosina <jkosina@suse.cz>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, David Laight <David.Laight@aculab.com>, Denys Vlasenko <dvlasenk@redhat.com>, Eduardo Valentin <eduval@amazon.com>, Greg KH <gregkh@linuxfoundation.org>, Will Deacon <will.deacon@arm.com>, "Liguori, Anthony" <aliguori@amazon.com>, Daniel Gruss <daniel.gruss@iaik.tugraz.at>, Hugh Dickins <hughd@google.com>, Kees Cook <keescook@google.com>, Andrea Arcangeli <aarcange@redhat.com>, Waiman Long <llong@redhat.com>, Joerg Roedel <jroedel@suse.de>

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@8bytes.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

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