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

[Paul E. McKenney]

On Sat, Aug 04, 2012 at 11:59:10PM +0100, Dr. David Alan Gilbert wrote:
> * 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.

Or the compiler could record a recovery address in a per-thread variable
before doing the speculative reference.  The page-fault handler could
consult the per-thread variable and take appropriate action.

But both this approach and your approach are vulnerable to things like
having the speculation area mapped to (say) MMIO space.  Not good!

So I am with Andrea on this one -- there would need to be some handshake
between kernel and compiler to avoid messing with possibly-unsafe
mappings.  And I am still not much in favor of value speculation.  ;-)

							Thanx, Paul

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