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    SubjectRe: [RFC] page-table walkers vs memory order
    On Sat, Aug 04, 2012 at 11:59:10PM +0100, Dr. David Alan Gilbert wrote:
    > * Andrea Arcangeli ( 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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