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Subject: Re: [PATCH 09/31] x86/entry/32: Leave the kernel via trampoline stack
To:     Joerg Roedel <jroedel@suse.de>,
        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>,
        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>,
        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>
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 <20180209190226.lqh6twf7thfg52cq@suse.de>
From:   Denys Vlasenko <dvlasenk@redhat.com>
Message-ID: <4a047ea5-7717-d089-48bf-597434be7c4c@redhat.com>
Date:   Fri, 9 Feb 2018 20:30:55 +0100
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On 02/09/2018 08:02 PM, Joerg Roedel wrote:
> On Fri, Feb 09, 2018 at 09:05:02AM -0800, Linus Torvalds 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.
>
> Okay, I used movsb because I remembered that being the recommendation
> for the most efficient memcpy, and it safes me an instruction. But that
> is probably only true on modern CPUs.

It's fast (copies data with full-width loads and stores,
up to 64-byte wide on latest Intel CPUs), but this kicks in only for
largish blocks. In your case, you are copying less than 100 bytes.