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From: Thomas Gleixner <tglx@linutronix.de>
To: "Chang S. Bae" <chang.seok.bae@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>, Andy Lutomirski <luto@kernel.org>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Andi Kleen <ak@linux.intel.com>,
	Ravi Shankar <ravi.v.shankar@intel.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	x86@kernel.org
Subject: Re: [RESEND PATCH v6 04/12] x86/fsgsbase/64: Enable FSGSBASE instructions in the helper functions
Date: Mon, 25 Mar 2019 14:05:40 +0100 (CET)
Message-ID: <alpine.DEB.2.21.1903251401580.1656@nanos.tec.linutronix.de> (raw)
In-Reply-To: <alpine.DEB.2.21.1903251345180.1656@nanos.tec.linutronix.de>

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

  reply index

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-15 20:06 [RESEND PATCH v6 00/12] x86: Enable FSGSBASE instructions Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 01/12] x86/fsgsbase/64: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 02/12] kbuild: Raise the minimum required binutils version to 2.21 Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 03/12] x86/fsgsbase/64: Add intrinsics for FSGSBASE instructions Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 04/12] x86/fsgsbase/64: Enable FSGSBASE instructions in the helper functions Chang S. Bae
2019-03-25 11:38   ` Thomas Gleixner
2019-03-25 12:46     ` Thomas Gleixner
2019-03-25 13:05       ` Thomas Gleixner [this message]
2019-03-26  0:38     ` Andi Kleen
2019-03-26 15:01       ` New feature/ABI review process [was Re: [RESEND PATCH v6 04/12] x86/fsgsbase/64:..] Thomas Gleixner
2019-03-26 22:56         ` Andi Kleen
2019-03-27 21:15           ` Thomas Gleixner
2019-03-15 20:06 ` [RESEND PATCH v6 05/12] x86/fsgsbase/64: Preserve FS/GS state in __switch_to() if FSGSBASE is on Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 06/12] x86/fsgsbase/64: When copying a thread, use the FSGSBASE instructions if available Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 07/12] x86/fsgsbase/64: Introduce the FIND_PERCPU_BASE macro Chang S. Bae
2019-03-25  9:02   ` Thomas Gleixner
2019-05-01 13:52     ` Bae, Chang Seok
2019-03-15 20:06 ` [RESEND PATCH v6 08/12] x86/fsgsbase/64: Use the per-CPU base as GSBASE at the paranoid_entry Chang S. Bae
2019-03-25  9:44   ` Thomas Gleixner
2019-04-05  8:35     ` Thomas Gleixner
2019-04-05 13:50       ` Andy Lutomirski
2019-05-01 13:52         ` Bae, Chang Seok
2019-05-01 17:40           ` Andy Lutomirski
2019-05-01 18:01             ` Bae, Chang Seok
     [not found]               ` <7029A32B-958E-4C1E-8B5F-D49BA68E4755@intel.com>
2019-05-01 20:25                 ` Andy Lutomirski
2019-05-01 21:04                   ` Bae, Chang Seok
2019-05-02  0:29                     ` Andy Lutomirski
2019-05-06 22:56     ` Bae, Chang Seok
2019-03-15 20:06 ` [RESEND PATCH v6 09/12] selftests/x86/fsgsbase: Test WRGSBASE Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 10/12] x86/fsgsbase/64: Enable FSGSBASE by default and add a chicken bit Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 11/12] x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2 Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 12/12] x86/fsgsbase/64: Add documentation for FSGSBASE Chang S. Bae
2019-03-30 16:15   ` Randy Dunlap
2019-03-26  0:43 ` [RESEND PATCH v6 00/12] x86: Enable FSGSBASE instructions Andy Lutomirski

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