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

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Message-ID: <alpine.DEB.2.21.1903251401580.1656@nanos.tec.linutronix.de>
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