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From: Andy Lutomirski <luto@kernel.org>
Date: Fri, 9 Feb 2018 17:43:55 +0000
Message-ID: <CALCETrUfMk67QeubuujbzKXeJFT4hBcq6kuAX3r1bOOeU1bNSQ@mail.gmail.com>
Subject: Re: [PATCH 09/31] x86/entry/32: Leave the kernel via trampoline stack
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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>, 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>, 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>

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.

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