linux-kernel - Re: Linux 4.15-rc7

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Message-ID: <CA+55aFwS8fV5Ymg-OBqYwkNMk56_wf5NP9z5UUm6CkP8ib6M=Q@mail.gmail.com>
Date:   Fri, 12 Jan 2018 09:34:03 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Pavel Machek <pavel@....cz>, Olivier Galibert <galibert@...ox.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        jikos@...e.cz
Subject: Re: Linux 4.15-rc7

On Fri, Jan 12, 2018 at 5:23 AM, Arnd Bergmann <arnd@...db.de> wrote:
>
> However, there is an important difference between KPTI and X86_4G:
> The former unmaps the kernel pages from the user space page tables,
> but keeps both the linear mapping and the user pages visible in
> kernel mode, while the latter must have also unmapped user space
> pages from kernel mode, requiring a more expensive get_user/put_user
> implementation.

Indeed. And I think that the 4G:4G patches do things wrong.

People are already complaining about the PTI costs. Separating user
space entirely is much much worse, and makes all user accesses from
kernel space too painful for words.

Honestly, I didn't merge the old 4G:4G patches originally, and I'm not
going to merge them this time around either.

                 Linus