On Sep 17, 2014 1:46 AM, "H. Peter Anvin" <hpa@xxxxxxxxx> 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@xxxxxxxxxxxxxx> 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 > > -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
