Re: [RFC] page-table walkers vs memory order

From: Dr. David Alan Gilbert
Date: Sat Aug 04 2012 - 19:00:22 EST


* Andrea Arcangeli (aarcange@xxxxxxxxxx) wrote:
> On Sat, Aug 04, 2012 at 03:02:45PM -0700, Paul E. McKenney wrote:
> > OK, I'll bite. ;-)
>
> :))
>
> > The most sane way for this to happen is with feedback-driven techniques
> > involving profiling, similar to what is done for basic-block reordering
> > or branch prediction. The idea is that you compile the kernel in an
> > as-yet (and thankfully) mythical pointer-profiling mode, which records
> > the values of pointer loads and also measures the pointer-load latency.
> > If a situation is found where a given pointer almost always has the
> > same value but has high load latency (for example, is almost always a
> > high-latency cache miss), this fact is recorded and fed back into a
> > subsequent kernel build. This subsequent kernel build might choose to
> > speculate the value of the pointer concurrently with the pointer load.
> >
> > And of course, when interpreting the phrase "most sane way" at the
> > beginning of the prior paragraph, it would probably be wise to keep
> > in mind who wrote it. And that "most sane way" might have little or
> > no resemblance to anything that typical kernel hackers would consider
> > anywhere near sanity. ;-)
>
> I see. The above scenario is sure fair enough assumption. We're
> clearly stretching the constraints to see what is theoretically
> possible and this is a very clear explanation of how gcc could have an
> hardcoded "guessed" address in the .text.
>
> Next step to clearify now, is how gcc can safely dereference such a
> "guessed" address without the kernel knowing about it.
>
> If gcc would really dereference a guessed address coming from a
> profiling run without kernel being aware of it, it would eventually
> crash the kernel with an oops. gcc cannot know what another CPU will
> do with the kernel pagetables. It'd be perfectly legitimate to
> temporarily move the data at the "guessed address" to another page and
> to update the pointer through stop_cpu during some weird "cpu
> offlining scenario" or anything you can imagine. I mean gcc must
> behave in all cases so it's not allowed to deference the guessed
> address at any given time.

A compiler could decide to dereference it using a non-faulting load,
do the calculations or whatever on the returned value of the non-faulting
load, and then check whether the load actually faulted, and whether the
address matched the prediction before it did a store based on it's
guess.

Dave
--
-----Open up your eyes, open up your mind, open up your code -------
/ Dr. David Alan Gilbert | Running GNU/Linux | Happy \
\ gro.gilbert @ treblig.org | | In Hex /
\ _________________________|_____ http://www.treblig.org |_______/
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/