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From: Thomas Gleixner <tglx@linutronix.de>
To: Florian Weimer <fweimer@redhat.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@kernel.org>,
	x86@kernel.org, Peter Zijlstra <peterz@infradead.org>,
	Matt Rickard <matt@softrans.com.au>,
	Stephen Boyd <sboyd@kernel.org>,
	John Stultz <john.stultz@linaro.org>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	devel@linuxdriverproject.org,
	virtualization@lists.linux-foundation.org,
	Paolo Bonzini <pbonzini@redhat.com>,
	Arnd Bergmann <arnd@arndb.de>, Juergen Gross <jgross@suse.com>
Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support
Date: Fri, 14 Sep 2018 15:19:24 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1809141514180.10480@nanos.tec.linutronix.de> (raw)
In-Reply-To: <63a0f67d-fdb1-e2fc-5d4d-4f3cfbf86fd1@redhat.com>

On Fri, 14 Sep 2018, Florian Weimer wrote:

> On 09/14/2018 03:05 PM, Thomas Gleixner wrote:
> > On Fri, 14 Sep 2018, Florian Weimer wrote:
> > 
> > > On 09/14/2018 02:50 PM, Thomas Gleixner wrote:
> > > > Matt attempted to add CLOCK_TAI support to the VDSO clock_gettime()
> > > > implementation, which extended the clockid switch case and added yet
> > > > another slightly different copy of the same code.
> > > > 
> > > > Especially the extended switch case is problematic as the compiler tends
> > > > to
> > > > generate a jump table which then requires to use retpolines.
> > > 
> > > Does vDSO code really have to use retpolines?  It's in userspace, after
> > > all.
> > 
> > Unless you have IBRS/STIPB enabled, you need user space ratpoutine as well.
> 
> I don't think this is a consensus position, and it obviously depends on the
> (sub)architecture.

It does, but we are not building kernels for specific micro architectures
nor do distros AFAIK.

But that aside, even with jump tables the generated code (ratpoutine
disabled) is not any better than with the current approach. It's even worse
and needs the same optimizations at the end and then the main difference is
that you have 3 copies of one function and 2 of the other. Not a win at
all.

Thanks,

	tglx



  reply	other threads:[~2018-09-14 13:19 UTC|newest]

Thread overview: 81+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-14 12:50 [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support Thomas Gleixner
2018-09-14 12:50 ` [patch 01/11] clocksource: Provide clocksource_arch_init() Thomas Gleixner
2018-09-14 12:50 ` [patch 02/11] x86/time: Implement clocksource_arch_init() Thomas Gleixner
2018-09-14 15:45   ` Vitaly Kuznetsov
2018-09-15  6:05     ` Thomas Gleixner
2018-09-14 12:50 ` [patch 03/11] x86/vdso: Enforce 64bit clocksource Thomas Gleixner
2018-09-14 12:50 ` [patch 04/11] x86/vdso: Use unsigned int consistently for vsyscall_gtod_data::seq Thomas Gleixner
2018-09-14 12:50 ` [patch 05/11] x86/vdso: Introduce and use vgtod_ts Thomas Gleixner
2018-09-14 12:50 ` [patch 06/11] x86/vdso: Collapse high resolution functions Thomas Gleixner
2018-09-14 12:50 ` [patch 07/11] x86/vdso: Collapse coarse functions Thomas Gleixner
2018-09-14 12:50 ` [patch 08/11] x86/vdso: Replace the clockid switch case Thomas Gleixner
2018-09-14 12:50 ` [patch 09/11] x86/vdso: Simplify the invalid vclock case Thomas Gleixner
2018-09-17 19:25   ` Andy Lutomirski
2018-09-17 20:12     ` John Stultz
2018-09-18  7:52       ` Thomas Gleixner
2018-09-18  8:30         ` Peter Zijlstra
2018-09-18  8:52           ` Thomas Gleixner
2018-09-18 10:06             ` Thomas Gleixner
2018-09-18 10:41               ` Thomas Gleixner
2018-09-18 12:48                 ` Peter Zijlstra
2018-09-18 13:23                   ` Thomas Gleixner
2018-09-18 13:38                     ` Peter Zijlstra
2018-09-18 15:52                     ` Thomas Gleixner
2018-09-27 14:41                       ` Thomas Gleixner
2018-09-18 14:01         ` Andy Lutomirski
2018-09-18 22:46           ` Thomas Gleixner
2018-09-18 23:03             ` Andy Lutomirski
2018-09-18 23:16               ` Thomas Gleixner
2018-09-27 14:36                 ` Thomas Gleixner
2018-09-27 14:39                   ` Andy Lutomirski
2018-09-19  9:08             ` Rasmus Villemoes
2018-09-19 13:29               ` Thomas Gleixner
2018-09-14 12:50 ` [patch 10/11] x86/vdso: Move cycle_last handling into the caller Thomas Gleixner
2018-09-14 15:26   ` Vitaly Kuznetsov
2018-09-14 12:50 ` [patch 11/11] x66/vdso: Add CLOCK_TAI support Thomas Gleixner
2018-09-14 14:04   ` Andy Lutomirski
2018-09-14 14:27     ` Thomas Gleixner
2018-09-14 14:59       ` Andy Lutomirski
2018-09-16  9:39         ` Thomas Gleixner
2018-09-14 12:56 ` [patch 00/11] x86/vdso: Cleanups, simmplifications and " Florian Weimer
2018-09-14 13:05   ` Thomas Gleixner
2018-09-14 13:06     ` Florian Weimer
2018-09-14 13:19       ` Thomas Gleixner [this message]
2018-09-14 13:09   ` Peter Zijlstra
2018-09-14 14:22 ` Arnd Bergmann
2018-09-17 13:00   ` Thomas Gleixner
2018-09-24 21:08     ` Arnd Bergmann
2018-10-03  5:15 ` Andy Lutomirski
2018-10-03  9:22   ` Vitaly Kuznetsov
2018-10-03 10:20     ` Andy Lutomirski
2018-10-03 12:01       ` Vitaly Kuznetsov
2018-10-03 14:20         ` Andy Lutomirski
2018-10-03 15:10           ` Thomas Gleixner
2018-10-03 16:18             ` Andy Lutomirski
2018-10-03 19:06     ` Marcelo Tosatti
2018-10-04  7:54       ` Vitaly Kuznetsov
2018-10-04  8:11         ` Peter Zijlstra
2018-10-04 14:00           ` Andy Lutomirski
2018-10-04 19:31             ` Peter Zijlstra
2018-10-04 20:05               ` Andy Lutomirski
2018-10-04 22:15                 ` Andy Lutomirski
2018-10-06 20:27                   ` Marcelo Tosatti
2018-10-06 22:28                     ` Andy Lutomirski
2018-10-08 15:26                       ` Marcelo Tosatti
2018-10-08 17:38                         ` Andy Lutomirski
2018-10-08 19:36                           ` Marcelo Tosatti
2018-10-09 20:09                             ` Andy Lutomirski
2018-10-11 22:27                               ` Marcelo Tosatti
2018-10-11 23:00                                 ` Andy Lutomirski
2018-10-15 13:39                                   ` Marcelo Tosatti
2018-10-06 20:49                   ` Marcelo Tosatti
2018-10-04 12:00         ` Paolo Bonzini
2018-10-04 14:04           ` Andy Lutomirski
2018-10-05 21:18         ` Marcelo Tosatti
2018-10-03 19:00   ` Marcelo Tosatti
2018-10-03 19:05     ` [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support\ Marcelo Tosatti
2018-10-03 22:32     ` [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support Andy Lutomirski
2018-10-04 16:37       ` Marcelo Tosatti
2018-10-04 17:08         ` Andy Lutomirski
2018-10-04 17:28           ` Vitaly Kuznetsov
2018-10-04 20:32 ` Andy Lutomirski

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