linux-kernel - Re: [RESEND PATCH v6 04/12] x86/fsgsbase/64: Enable FSGSBASE instructions in the helper functions

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]

Date:   Mon, 25 Mar 2019 14:05:40 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>
cc:     Ingo Molnar <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Ravi Shankar <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>, x86@...nel.org
Subject: Re: [RESEND PATCH v6 04/12] x86/fsgsbase/64: Enable FSGSBASE
 instructions in the helper functions

On Mon, 25 Mar 2019, Thomas Gleixner wrote:

> On Mon, 25 Mar 2019, Thomas Gleixner wrote:
> > The whole point of FSGSBASE support is performance, right?
> > 
> > So can please someone explain why having the following in the context
> > switch path when it can be completely avoided is enhancing performance:
> > 
> >   - 4 x SWAPGS
> >   - 1 x RDMSR
> >   - 1 x WRMSR
> 
> Corrrecting myself. That should be:
> 
>      RDGSBASE
>      WRGSBASE
> 
> obviously. Still the point remains.
> 
> >   - 2 x local_irq_save()
> >   - 2 x local_irq_restore()
> > 
> > Of course the local_irq_save/restore() pairs are utterly pointless because
> > switch_to() runs with interrupts disabled already.
> > 
> > SWAPGS instead needs:
> > 
> >   1 x WRMSR
> > 
> > and nothing else.
> > 
> > So trading the single WRMSR against the above in the context switch path is
> > gaining performance, right?

And even IF the sequences are faster than the single WRMSR, this does not
justify the mixed bag of SWAPGS/FSGSBASE usage at all.

Thanks,

	tglx