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    SubjectRe: [PATCH 2/5] ftrace: use code patching for ftrace graph tracer
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    On Tue, 2008-11-25 at 21:35 -0800, Andrew Morton wrote:
    > On Wed, 26 Nov 2008 00:16:24 -0500 Steven Rostedt <rostedt@goodmis.org> wrote:
    >
    > > From: Steven Rostedt <rostedt@goodmis.org>
    > >
    > > Impact: more efficient code for ftrace graph tracer
    > >
    > > This patch uses the dynamic patching, when available, to patch
    > > the function graph code into the kernel.
    > >
    > > This patch will ease the way for letting both function tracing
    > > and function graph tracing run together.
    > >
    > > ...
    > >
    > > +static int ftrace_mod_jmp(unsigned long ip,
    > > + int old_offset, int new_offset)
    > > +{
    > > + unsigned char code[MCOUNT_INSN_SIZE];
    > > +
    > > + if (probe_kernel_read(code, (void *)ip, MCOUNT_INSN_SIZE))
    > > + return -EFAULT;
    > > +
    > > + if (code[0] != 0xe9 || old_offset != *(int *)(&code[1]))
    >
    > erk. I suspect that there's a nicer way of doing this amongst our
    > forest of get_unaligned_foo() interfaces. Harvey will know.
    >

    if (code[0] != 0xe9 || old_offset != get_unaligned((int *)(&code[1])))

    > > + return -EINVAL;
    > > +
    > > + *(int *)(&code[1]) = new_offset;
    >
    > Might be able to use put_unaligned_foo() here.
    >

    put_unaligned(new_offset, (int *)(&code[1]));

    > The problem is that these functions use sizeof(*ptr) to work out what
    > to do, so a cast is still needed. A get_unaligned32(ptr) would be
    > nice. One which takes a void* and assumes CPU ordering.

    I've been thinking similarly, I could investigate something that
    goes in with the _noalign stuff?

    I'll finish the documentation patch for the _noalign stuff and then see
    about doing the host-order bits to fit in as well.

    Harvey



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