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    SubjectRe: mmiotrace bug: recursive probe hit
    On Thu, Apr 3, 2008 at 11:07 PM, Pekka Paalanen <> wrote:
    > Pekka Paalanen <> wrote:
    > > C) Vegard mentioned something about per-cpu page tables for kmemcheck.
    > > This would be the ultimate solution, because it would solve two problems:
    > > - recursive probe hits
    > > - missed events due to another cpu disarming the page for single stepping
    > > Would it be possible to have a single temporary per-cpu pte?
    > >
    > > I understood kmemcheck has similar issues. Of course, one could force the
    > > system down to a single running CPU, but that feels nasty.

    Yes, Ingo Molnar has suggested per-cpu page tables, but that's so far
    away from what I am capable of, so unless Ingo wants to do it himself,
    I fear it will never be done ;-) [I also believe the resulting code
    would be too ugly and too un-useful for the rest of the kernel that it
    would probably not ever be merged. But that's a different story.] But
    I do think this is the best solution in terms of reliability.

    We do indeed limit maxcpus to 1 at run-time if the kernel is compiled
    with CONFIG_SMP. kmemcheck is a debugging facility, and as such,
    actual multiprocessor support is not critical for the purpose of
    kmemcheck, in my opinion. Doesn't the same hold for mmiotrace?

    > One more idea:
    > D) Emulate the faulting instruction.
    > In __ioremap(), do the mapping, but steal it for mmiotrace's personal use,
    > and return a bogus mapping that is identifiable in #pf handler. When
    > something accesses the bogus mapping, emulate and step over the faulting
    > instruction using the stolen IO memory mapping. This would get rid of
    > the debug trap and single stepping, and also remove the need to disarm
    > the mmio page, which means tracing would work reliably on SMP without
    > any page table kludges. This would also remove the yet another instruction
    > decoding code that mmiotrace has.
    > The catch is the instruction emulation. I see KVM has some emulation code,
    > but I cannot understand it without a deep study that would take me weeks.
    > Is that general enough to be used, or could it be generalized?
    > Mmiotrace, apart from executing the instruction with a modified address,
    > would need to extract the type of IO memory access, width and the data
    > read/written. And since it is dealing with IO memory, the emulation
    > should be very careful to access the hardware exactly like the original
    > instruction would have.

    I think that would be extremely difficult to do. I am personally
    trying to stay as far away from opcode decoding (and recoding!
    *shudder*) as possible. I do the minimal decoding for operand sizes,
    etc, which I think you do as well in mmiotrace.

    > Maybe also kmemcheck could use this approach, since the current approach
    > is very much like in mmiotrace: #pf, show page, single step, #db trap,
    > hide page.

    They are indeed very much the same. I wish somebody had told me about
    mmiotrace when I first started working on kmemcheck! :-)

    I don't think I can be of much more help than that. Just my opinion on things.

    Kind regards,
    Vegard Nossum

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