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    SubjectRe: [RFC PATCH 1/2] x86: Fix rbp saving in pt_regs on irq entry
    On Fri, Jan 07, 2011 at 01:31:30PM +0100, Ingo Molnar wrote:
    >
    > * Jan Beulich <JBeulich@novell.com> wrote:
    >
    > > > Now I don't understand how this is all useful as this is not a normal proc but
    > > > an interruption. We can't get back the return address from the CFA. Or am I
    > > > missing something?
    > >
    > > Unwind annotations, when written correctly, allow unwinding through all kinds of
    > > execution flows, including interrupts or exceptions as well as including stack
    > > switches.
    >
    > Yeah and that's rather useful, as exception contexts can nest in very weird ways,
    > especially with NMIs involved. For example a 7-context combination is possible:
    >
    > user-space -> syscall -> pagefault -> softirq -> hardirq -> debug trap -> NMI
    >
    > And the call frame walking logic needs to be able to get all the way back to
    > user-space ...
    >
    > For that every transition needs to work flawlessly, for debugging (and CFI based
    > profiling) to work fine.
    >
    > Most of those transitions can happen at any instruction boundary that a given
    > context executes, so the total number of possible combinations is virtually endless.
    >
    > Unfortunately we dont seem to have a good way to test any of this automatically.
    > Putting a perf probe on every assembly instruction perhaps, and checking whether the
    > frame manages to go back all the way to user-space?

    May be.

    Once I'll have perf callchain based on CFI ready, we'll perhaps find some issues
    there. Although I guess there are already tools that can make use of that.

    >
    > Thanks,
    >
    > Ingo


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