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

["H. Peter Anvin" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org>]

On 09/16/2014 11:21 PM, Filipe Brandenburger wrote:
> Hi Andy,
> 
> On Tue, Sep 16, 2014 at 10:00 PM, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> 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