linux-kernel - Re: [PATCH] x86/vdso: Add prctl to set per-process VDSO load

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Message-ID: <54194A4A.7030104@zytor.com>
Date:	Wed, 17 Sep 2014 01:46:02 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Filipe Brandenburger <filbranden@...gle.com>,
	Andy Lutomirski <luto@...capital.net>
CC:	Richard Larocque <rlarocque@...gle.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michael Davidson <md@...gle.com>,
	Greg Thelen <gthelen@...gle.com>, X86 ML <x86@...nel.org>,
	Linux API <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
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 <luto@...capital.net> 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.

	-hpa


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