Date: Fri, 12 Jan 2018 20:38:15 +0100
From: Pavel Machek <pavel@ucw.cz>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>,
        Olivier Galibert <galibert@pobox.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        jikos@suse.cz
Subject: Re: Linux 4.15-rc7
Message-ID: <20180112193815.GA4918@amd>
References: <CA+55aFwbmKznuQZe_VemxXJPSmB-9yy8uyWsMZcrkA4T=m8xkw@mail.gmail.com>
 <20180110233252.GA14739@amd>
 <CAMFK0guEASKq1zS3S3u=e7TAAYTvmfWfWe5k+X+hKV6RTpyw_Q@mail.gmail.com>
 <20180112110624.GA13254@amd>
 <CAK8P3a3xs2-4VXKWum7cXrwUqK7Ysz3HOcikb53qQde2R6Ui4w@mail.gmail.com>
 <CA+55aFwS8fV5Ymg-OBqYwkNMk56_wf5NP9z5UUm6CkP8ib6M=Q@mail.gmail.com>
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha1;
        protocol="application/pgp-signature"; boundary="+HP7ph2BbKc20aGI"
Content-Disposition: inline
In-Reply-To: <CA+55aFwS8fV5Ymg-OBqYwkNMk56_wf5NP9z5UUm6CkP8ib6M=Q@mail.gmail.com>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org


--+HP7ph2BbKc20aGI
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

On Fri 2018-01-12 09:34:03, Linus Torvalds wrote:
> On Fri, Jan 12, 2018 at 5:23 AM, Arnd Bergmann <arnd@arndb.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.
>=20
> Indeed. And I think that the 4G:4G patches do things wrong.

Yeah. But if there's copy around for something recent, I'd still like
to see it.

> 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.
>=20
> Honestly, I didn't merge the old 4G:4G patches originally, and I'm not
> going to merge them this time around either.

I'll try to do the right thing. OTOH... I don't like the fact that
kernel memory on my machine is currently readable, probably even from
javascript.

I tried disabling CPU caches. Just like that, off, boom. My system
will not survive that, and it looks like 100x slowdown. So 2x slowdown
would be an improvement (and 4G:4G can probably do better than that).

									Pavel
--=20
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blo=
g.html

--+HP7ph2BbKc20aGI
Content-Type: application/pgp-signature; name="signature.asc"
Content-Description: Digital signature

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1

iEYEARECAAYFAlpZDqcACgkQMOfwapXb+vI/dACdFEGTRTdbEorVwOhkP1ffTBO+
tdwAniLjXx/eGmAGVEOwnX9d69zv7T65
=4AJq
-----END PGP SIGNATURE-----

--+HP7ph2BbKc20aGI--