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Subject: Re: [PATCH 09/31] x86/entry/32: Leave the kernel via trampoline stack
References: <1518168340-9392-1-git-send-email-joro@8bytes.org>
 <1518168340-9392-10-git-send-email-joro@8bytes.org>
 <CA+55aFzB9H=RT6YB3onZCephZMs9ccz4aJ_jcPcfEkKJD_YDCQ@mail.gmail.com>
From: Denys Vlasenko <dvlasenk@redhat.com>
Message-ID: <aa52108c-4874-9810-8ff5-e6415189cd73@redhat.com>
Date: Fri, 9 Feb 2018 18:17:07 +0100
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To: Linus Torvalds <torvalds@linux-foundation.org>, Joerg Roedel <joro@8bytes.org>
Cc: 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>, Joerg Roedel <jroedel@suse.de>

On 02/09/2018 06:05 PM, 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.

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.

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