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Date:	Wed, 17 Sep 2014 01:46:02 -0700
From:	"H. Peter Anvin" <>
To:	Filipe Brandenburger <>,
	Andy Lutomirski <>
CC:	Richard Larocque <>,
	Ingo Molnar <>,
	Thomas Gleixner <>,
	Michael Davidson <>,
	Greg Thelen <>, X86 ML <>,
	Linux API <>,
	"" <>
Subject: Re: [PATCH] x86/vdso: Add prctl to set per-process VDSO load

On 09/16/2014 11:21 PM, Filipe Brandenburger wrote:
> Hi Andy,
> On Tue, Sep 16, 2014 at 10:00 PM, Andy Lutomirski <> wrote:
>> I think that the patch should instead tweak the vvar mapping to tell
>> the vdso not to use rdtsc.  It should be based on this:
> I've been working on this approach which extends the vvar from 2 to 3
> pages. The third page would initially be mapped to a zero page but
> then through a prctl a task could replace it with a real page that
> could then be inherited through fork and exec.
> That would make it possible to have per-task vvar contents.
> We could use some of those values as flags to indicate whether vdso
> routines may use RDTSC or not.
> In the future, we're planning to also use that to store clock offsets
> so that we can ensure CLOCK_MONOTONIC works after CRIU migration
> without having to turn off the VDSO or have to always fallback to full
> syscalls on every case.
> Do you think that would be a reasonable way to accomplish that?

Why would we need/want per process vvar contents?  It seems better to
have the code swapped out.


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