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Re: [RFC][PATCH] kprobes: kprobe-booster for ia64
- From: Masami Hiramatsu <mhiramat at redhat dot com>
- To: "Luck, Tony" <tony dot luck at intel dot com>
- Cc: Ananth N Mavinakayanahalli <ananth at in dot ibm dot com>, Jim Keniston <jkenisto at us dot ibm dot com>, "Li, Shaohua" <shaohua dot li at intel dot com>, LKML <linux-kernel at vger dot kernel dot org>, ia64 <linux-ia64 at vger dot kernel dot org>, systemtap-ml <systemtap at sources dot redhat dot com>
- Date: Wed, 06 Feb 2008 07:51:47 -0500
- Subject: Re: [RFC][PATCH] kprobes: kprobe-booster for ia64
- References: <47A8E995.1090400@redhat.com> <1FE6DD409037234FAB833C420AA843EC84804C@orsmsx424.amr.corp.intel.com>
Hello Tony,
Thank you for review.
Luck, Tony wrote:
> +/* Insert a long branch code */
> +static void __kprobes set_brl_inst(void *from, void *to)
> +{
> + s64 rel = ((s64) to - (s64) from) >> 4;
> + bundle_t *brl;
> + brl = (bundle_t *) ((u64) from & ~0xf);
> + brl->quad0.template = 0x05; /* [MLX](stop) */
> + brl->quad0.slot0 = NOP_M_INST; /* nop.m 0x0 */
> + brl->quad0.slot1_p0 = ((rel >> 20) & 0x7fffffffff) << 2;
> + brl->quad1.slot1_p1 = (((rel >> 20) & 0x7fffffffff) << 2) >> (64 - 46);
> + /* brl.cond.sptk.many.clr rel<<4 (qp=0) */
> + brl->quad1.slot2 = BRL_INST(rel >> 59, rel & 0xfffff);
> +}
>
> Is this function operating directly on the instruction bundle in the
> kernel code? If so, the bundle will contain some "interesting"
> intermediate values as each of the "brl->xxx = yyy;" are processed.
> If another cpu should try to execute this bundle while the modifications
> are in progress, then the results will be not what you want.
No, this function writes brl on kprobe's instruction buffer
(kprobe.ainsn.insn->bundle[1]) when preparing the kprobe.
So, other cpu never execute it, and we don't need to write it atomically.
> On cpu models that support st16 you could construct the brl instruction
> and use a single st16 to update the whole bundle atomically.
>
> -Tony
Thank you again,
--
Masami Hiramatsu
Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division
e-mail: mhiramat@redhat.com