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Date: Mon, 22 Jan 2018 11:11:18 +0100
From: Joerg Roedel <joro@8bytes.org>
Subject: Re: [PATCH 02/16] x86/entry/32: Enter the kernel via trampoline stack
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To: Andy Lutomirski <luto@kernel.org>
Cc: 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>, Brian Gerst <brgerst@gmail.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>

Hey Andy,

On Fri, Jan 19, 2018 at 08:30:33AM -0800, Andy Lutomirski wrote:
> I meant that we could have sp0 have a genuinely constant value per
> cpu.  That means that the entry trampoline ends up with RIP, etc in a
> different place depending on whether VM was in use, but the entry
> trampoline code should be able to handle that.  sp1 would have a value
> that varies by task, but it could just point to the top of the stack
> instead of being changed depending on whether VM is in use.  Instead,
> the entry trampoline would offset the registers as needed to keep
> pt_regs in the right place.
> 
> I think you already figured all of that out, though :)

Yes, and after looking a while into it, it would make a nice cleanup for
the entry code. On the other side, it would change the layout for the
in-kernel 'struct pt_regs', so that the user-visible pt_regs ends up
with a different layout than the one we use in the the kernel.

This can certainly be all worked out, but it makes this nice entry-code
cleanup not so nice and clean anymore. At least the work required to
make it work without breaking user-space is not in the scope of this
patch-set.


Regards,

	Joerg

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