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    SubjectRe: mmiotrace bug: recursive probe hit
    Pekka Paalanen wrote:
    > On Sun, 30 Mar 2008 20:26:08 +0300
    > 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.
    > One more idea:
    > 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.
    > 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.
    > Are there other x86(_64) instruction emulation facilities in the kernel
    > I might use?
    > Or, if the emulation cannot be used, what would it take to make at least
    > instruction decoding general enough so that mmiotrace could use that instead
    > of its own decoding?
    > I fear modifying KVM emulation code is a too heavy job for me personally.

    It should not be too difficult to modify x86_emulate.c to do everything
    through a function vector. However there is a simpler (for you)
    solution: run the driver-to-be-reverse-engineered in a kvm guest, and
    modify kvm userspace to log accesses to mmio regions. This requires the
    not-yet-merged pci passthrough support. You can reverse engineer
    Windows drivers with this as well.

    This won't work for kmemcheck smp though.

    Any sufficiently difficult bug is indistinguishable from a feature.

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