linux-kernel - Re: [PATCH 07/39] x86/entry/32: Enter the kernel via trampoline stack

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Date:   Thu, 19 Jul 2018 22:52:04 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Brian Gerst <brgerst@...il.com>
cc:     Joerg Roedel <joro@...tes.org>, Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.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 Kroah-Hartman <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>, Pavel Machek <pavel@....cz>,
        dhgutteridge@...patico.ca, Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 07/39] x86/entry/32: Enter the kernel via trampoline
 stack

On Wed, 18 Jul 2018, Brian Gerst wrote:
> > +.Lcopy_pt_regs_\@:
> > +#endif
> > +
> > +       /* Allocate frame on task-stack */
> > +       subl    %ecx, %edi
> > +
> > +       /* Switch to task-stack */
> > +       movl    %edi, %esp
> > +
> > +       /*
> > +        * We are now on the task-stack and can safely copy over the
> > +        * stack-frame
> > +        */
> > +       shrl    $2, %ecx
> 
> This shift can be removed if you divide the constants by 4 above.
> Ditto on the exit path in the next patch.

No, the

> > +       /* Allocate frame on task-stack */
> > +       subl    %ecx, %edi

needs the full value in bytes ....

Thanks,

	tglx