linux-kernel - Re: [PATCH] x86: branchless clear

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Message-ID: <CACVxJT8j3-DEcpSXoq2OSJ-carHMybSUZXn1D3xtqjK0TnCe3w@mail.gmail.com>
Date:   Tue, 23 Aug 2016 13:46:41 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: branchless clear_page()

On Tue, Aug 23, 2016 at 12:06 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Mon, Aug 22, 2016 at 11:52:29PM +0300, Alexey Dobriyan wrote:
>> Apply alternatives at the call site instead of function body.
>> Save branch per clean page.
>
> Any size increase to an allyesconfig vmlinux and increase to
> alternatives application time?
>
> Booting with "debug-alternative" should help.

x86_64 allyesconfig kernel lists whole 56 calls to clear_page().
My usual kernel lists 8.

There are tons copy_from_user/copy_to_user entries and POPCNT
and CLAC/STAC, so I think those several dozens clear pages don't matter.