linux-kernel - Re: [PATCH 09/31] x86/entry/32: Leave the kernel via trampoline stack

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Message-ID: <aa52108c-4874-9810-8ff5-e6415189cd73@redhat.com>
Date:   Fri, 9 Feb 2018 18:17:07 +0100
From:   Denys Vlasenko <dvlasenk@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Joerg Roedel <joro@...tes.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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>, 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>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <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>,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 09/31] x86/entry/32: Leave the kernel via trampoline stack

On 02/09/2018 06:05 PM, Linus Torvalds wrote:
> On Fri, Feb 9, 2018 at 1:25 AM, Joerg Roedel <joro@...tes.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.

Indeed, "rep movs" has some setup overhead that makes it undesirable
for small sizes. In my testing, moving less than 128 bytes with "rep movs"
is a loss.