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From:   Andy Lutomirski <luto@kernel.org>
Date:   Fri, 9 Feb 2018 17:43:55 +0000
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Subject: Re: [PATCH 09/31] x86/entry/32: Leave the kernel via trampoline stack
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        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>, Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
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        Peter Zijlstra <peterz@infradead.org>,
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        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>, Pavel Machek <pavel@ucw.cz>,
        Joerg Roedel <jroedel@suse.de>
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On Fri, Feb 9, 2018 at 5:05 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Fri, Feb 9, 2018 at 1:25 AM, Joerg Roedel <joro@8bytes.org> wrote:
>> +
>> +       /* Copy over the stack-frame */
>> +       cld
>> +       rep movsb
>
> Ugh. This is going to be horrendous. Maybe not noticeable on modern
> CPU's, but the whole 32-bit code is kind of pointless on a modern CPU.
>
> At least use "rep movsl". If the kernel stack isn't 4-byte aligned,
> you have issues.
>

The 64-bit code mostly uses a bunch of push instructions for this.