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    SubjectRe: [patch 02/11] x86 architecture implementation of Hardware Breakpoint interfaces

    * Alan Stern <> wrote:

    > On Sat, 14 Mar 2009, K.Prasad wrote:
    > > Here's a summary of the intended changes to the patchset, which I hope
    > > to post early the following week. It tears down many features in the
    > > present submission (The write-up below is done without the benefit of
    > > actually having run into limitations while trying to chisel out code).
    > >
    > > - Adopt a static allocation method for registers, say FCFS (and perhaps
    > > botton-up for user-space allocations and the reverse for
    > > kernel-space), although individual counters to do book-keeping should also
    > > suffice.
    > You can't enforce bottom-up allocation for userspace breakpoint
    > requests. [...]

    That's not the point.

    The point is to offer a reasonable and simple static allocator
    that will work fine with usual gdb usage. If something takes
    away db4 that's as if user-space took away all registers - tough

    You are trying to put complexity into a situation that is not
    schedulable hence not resolvable _anyway_. There's just 4 debug
    registers, not more. If the combined usage goes above four
    someone will lose anyway - even with your allocator.

    With my proposal the 'loss' can indeed come sooner if user-space
    took db4 and there's nothing left for the kernel anymore - but
    that's just an uninteresting special case that wont occur with
    typical debug-register usage.

    If it ever causes problems seriously _then_ will be the time to
    consider "is it worth adding a more complex, dynamic allocator
    for debug registers". Not now. This stuff is currently
    over-designed and not acceptable to me in its current form.


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