On Wed, Sep 17, 2014 at 7:28 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > 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. I have this (special mapping tracking) 3/4 implemented. I'm planning on making it fully functional for 64-bit programs and almost correct for 32-bit. (You'll still crash if you have multiple threads, you use sysenter, and you remap the vdso, but I think that this is essentially unavoidable until someone lets mremap work on multiple vmas at once.) > > 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 >> >> -- Andy Lutomirski AMA Capital Management, LLC -- 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
