linux-kernel - Re: [PATCH v4 1/7] x86, mm, pat: Set WT to PA7 slot of PAT MSR

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <alpine.DEB.2.11.1411031913140.5308@nanos>
Date:	Mon, 3 Nov 2014 19:15:17 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andy Lutomirski <luto@...capital.net>
cc:	Toshi Kani <toshi.kani@...com>, "H. Peter Anvin" <hpa@...or.com>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Juergen Gross <jgross@...e.com>,
	Stefan Bader <stefan.bader@...onical.com>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Yigal Korman <yigal@...xistor.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH v4 1/7] x86, mm, pat: Set WT to PA7 slot of PAT MSR

On Mon, 3 Nov 2014, Andy Lutomirski wrote:
> On Mon, Nov 3, 2014 at 9:47 AM, Toshi Kani <toshi.kani@...com> wrote:
> > On Mon, 2014-11-03 at 18:14 +0100, Thomas Gleixner wrote:
> >> On Mon, 27 Oct 2014, Toshi Kani wrote:
> >> > +   } else {
> >> > +           /*
> >> > +            * PAT full support. WT is set to slot 7, which minimizes
> >> > +            * the risk of using the PAT bit as slot 3 is UC and is
> >> > +            * currently unused. Slot 4 should remain as reserved.
> >>
> >> This comment makes no sense. What minimizes which risk and what has
> >> this to do with slot 3 and slot 4?
> >
> > This is for precaution.  Since the patch enables the PAT bit the first
> > time, it was suggested that we keep slot 4 reserved and set it to WB.
> > The PAT bit still has no effect to slot 0/1/2 (WB/WC/UC-) after this
> > patch.  Slot 7 is the safest slot since slot 3 (UC) is unused today.
> >
> > https://lkml.org/lkml/2014/9/4/691
> > https://lkml.org/lkml/2014/9/5/394
> >
> 
> I would clarify the comment, since this really has nothing to do with
> slot 3 being unused.  How about:
> 
> We put WT in slot 7 to improve robustness in the presence of errata
> that might cause the high PAT bit to be ignored.  This way a buggy
> slot 7 access will hit slot 3, and slot 3 is UC, so at worst we lose
> performance without causing a correctness issue.  Pentium 4 erratum
> N46 is an example of such an erratum, although we try not to use PAT
> at all on affected CPUs.

Indeed. That makes a lot more sense.
 
> >> > +            *
> >> > +            *  PTE encoding used in Linux:
> >> > +            *      PAT
> >> > +            *      |PCD
> >> > +            *      ||PWT  PAT
> >> > +            *      |||    slot
> >> > +            *      000    0    WB : _PAGE_CACHE_MODE_WB
> >> > +            *      001    1    WC : _PAGE_CACHE_MODE_WC
> >> > +            *      010    2    UC-: _PAGE_CACHE_MODE_UC_MINUS
> >> > +            *      011    3    UC : _PAGE_CACHE_MODE_UC
> >> > +            *      100    4    <reserved>
> >> > +            *      101    5    <reserved>
> >> > +            *      110    6    <reserved>
> >>
> >> Well, they are still mapped to WB/WC/UC_MINUS ....
> >
> > Right, the reserved slots are also initialized with their safe values.
> > However, the macros _PAGE_CACHE_MODE_XXX only refer to the slots
> > specified above.

Then the table should reflect this, i.e.: reserved, but mapped to XX

And a comment below that explaining WHY we map the reserved slots.

Thanks,

	tglx
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/