linux-kernel - Re: [RFC 2/5] x86: Store a per-cpu shadow copy of CR4

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: <CALCETrUxKqNnJQWQUbR-MSzEPj+cvxYgwBA9k9=1n_JOMAx1Ag@mail.gmail.com>
Date:	Thu, 16 Oct 2014 08:30:46 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Erik Bosman <ebn310@....vu.nl>
Subject: Re: [RFC 2/5] x86: Store a per-cpu shadow copy of CR4

On Oct 16, 2014 4:49 AM, "Borislav Petkov" <bp@...en8.de> wrote:
>
> On Tue, Oct 14, 2014 at 03:57:36PM -0700, Andy Lutomirski wrote:
> > Context switches and TLB flushes can change individual bits of CR4.
> > CR4 reads take several cycles, so store a shadow copy of CR4 in a
> > per-cpu variable.
> >
> > To avoid wasting a cache line, I added the CR4 shadow to
> > cpu_tlbstate, which is already touched during context switches.
>
> So does this even show in any workloads as any improvement?
>

Unlear.  Assuming that mov from cr4 is more expensive than a cache
miss (which may or may not be true), then kernel TLB flushes will get
cheaper.  The main reason I did this is to make switching TSD and PCE
a little cheaper, which might be worthwhile.

I think this may be a huge win on many workloads when running as an
SVM guest.  IIUC SVM doesn't have accelerated guest CR4 access.

> Also, what's the rule with reading the shadow CR4? kvm only? Because
> svm_set_cr4() in svm.c reads the host CR4 too.

Whoops.

>
> Should we make all code access the shadow CR4 maybe...

That was the intent.  In v2, I'll probably rename read_cr4 to
__read_cr4 to make it difficult to miss things.

>
> --
> Regards/Gruss,
>     Boris.
> --
--
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/