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

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Message-ID: <CADU+-uCguxhkw259MefABuzOxvyK_F6bhwza0YhveCCJxEV8wQ@mail.gmail.com>
Date:	Tue, 16 Sep 2014 23:21:06 -0700
From:	Filipe Brandenburger <filbranden@...gle.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Richard Larocque <rlarocque@...gle.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, 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

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?

Thanks,
Filipe
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