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    SubjectRe: [RFC Patch 2/9] x86 architecture implementation of Hardware Breakpoint interfaces
    On Tue, Oct 07, 2008 at 11:36:30AM -0400, Alan Stern wrote:
    > On Tue, 7 Oct 2008, K.Prasad wrote:
    >
    > > This patch introduces two new files named hw_breakpoint.[ch] inside x86 specific
    > > directories. They contain functions which help validate and serve requests for
    > > using Hardware Breakpoint registers on x86 processors.
    >
    > > --- /dev/null
    > > +++ linux-bkpt-lkml-27-rc9/arch/x86/kernel/hw_breakpoint.c
    > > @@ -0,0 +1,684 @@
    >
    > ...
    > > +int pre_handler_allowed(unsigned type)
    > > +{
    > > + if (type == HW_BREAKPOINT_EXECUTE)
    > > + return 1;
    > > + else
    > > + return -EINVAL;
    > > +}
    >
    > The routine's name should match the name in the header file. "allowed"
    > isn't right: You're _allowed_ to have pre_handlers -- they just won't
    > get invoked. "supported" would be better.

    pre_handler_supported() definitely sounds better. I will change them.

    >
    > Also, the comment in the header file should explain the meaning of the
    > return value -- you should return 0 if a pre_handler is not supported,
    > not -EINVAL. Better yet, define the function (both here and in the
    > header file) as returning bool rather than int.
    >

    I will change them to boolean.

    > > +
    > > +int post_handler_allowed(unsigned type)
    > > +{
    > > + /* We can have a post handler for all types of breakpoints */
    > > + return 1;
    > > +}
    >
    > Same comments as above.
    >
    > Also, in this initial version I would prefer to avoid the complications
    > of single-stepping. It can always be added later. So for now, the x86
    > implementation should not support post_handlers for execution
    > breakpoints.
    >

    There's been a perceivable inclination to let the user learn the
    limitations/features of the underlying processor's breakpointing ability
    (since the previous email mail thread on this topic) and the routines
    pre_ and post_handler_allowed() are just a step towards that.

    I can nullify the post_handler for x86-instruction breakpoint for now,
    but it wouldn't simplify things very extensively (but for a few lines of
    code in hw_breakpoint_handler() and the flag 'sstep_reason'). It also
    benefits the code by bringing an understanding that there can be
    multiple users of processor single-stepping (and therefore the need to
    de-multiplex the exception and invoke the appropriate handler).

    Left to me, I would like to retain the post_handler routine, unless you
    strongly feel otherwise.

    > ...
    > > +/*
    > > + * Validate the arch-specific HW Breakpoint register settings
    > > + */
    > > +static int arch_validate_hwbkpt_settings(struct hw_breakpoint *bp,
    > > + unsigned long address, unsigned len, unsigned int type,
    > > + unsigned int *align)
    >
    > Why did you move this routine into the arch-specific code?
    >
    > ...
    > > +/*
    > > + * Handle debug exception notifications.
    > > + */
    > > +
    > > +static void switch_to_none_hw_breakpoint(void);
    > > +struct hw_breakpoint *last_hit_bp;
    > > +struct thread_hw_breakpoint *last_hit_thbi;
    >
    > Shouldn't these variables be static? Although if they're needed only for
    > single-stepping, they can be removed entirely for now...
    >
    Agreed. Will make them static.

    Thanks,
    K.Prasad



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