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

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Date:	Wed, 17 Sep 2014 07:28:07 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Greg Thelen <gthelen@...gle.com>, X86 ML <x86@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michael Davidson <md@...gle.com>,
	Ingo Molnar <mingo@...hat.com>,
	Richard Larocque <rlarocque@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Filipe Brandenburger <filbranden@...gle.com>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] x86/vdso: Add prctl to set per-process VDSO load

On Sep 17, 2014 1:46 AM, "H. Peter Anvin" <hpa@...or.com> wrote:
>
> 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.

That seems messier from a build perspective.  Also, if we ever want to
switch this dynamically, swapping out data is much easier than
swapping out code.  I think we should be able to replace the vvar page
with the zero page, though.

One tricky bit: currently we can only easily do this on exec, but we
should be able to do it immediately if we start tracking mremap of the
vdso.  Should we make that a prerequisite?  I don't really want this
to end up being permanently weird.

As for an actual post-migration offset, I'd rather add support for
per-mm forced syscall fallback and then get something into the code
timing code before even thinking about an x86 vdso fast path.  I don't
think that a feature like per-mm timing offsets should happen as an
arch-specific thing first.

--Andy

>
>         -hpa
>
>
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