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 [off-list ref] wrote:

quoted

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