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Date: Thu, 19 Jul 2018 22:52:04 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 07/39] x86/entry/32: Enter the kernel via trampoline
 stack
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References: <1531906876-13451-1-git-send-email-joro@8bytes.org> <1531906876-13451-8-git-send-email-joro@8bytes.org> <CAMzpN2gqxu7rgVj8rfweanLNgHBci+nqZMqEYpvgRUd1828umQ@mail.gmail.com>
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To: Brian Gerst <brgerst@gmail.com>
Cc: Joerg Roedel <joro@8bytes.org>, Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>, the arch/x86 maintainers <x86@kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>, Linus Torvalds <torvalds@linux-foundation.org>, Andy Lutomirski <luto@kernel.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 Kroah-Hartman <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>, Pavel Machek <pavel@ucw.cz>, dhgutteridge@sympatico.ca, Joerg Roedel <jroedel@suse.de>

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