Messages in this thread | | | Date | Wed, 4 May 2022 13:43:26 +0100 | From | Mark Rutland <> | Subject | Re: [RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically allocated trampolines |
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On Thu, Apr 21, 2022 at 01:06:48PM -0400, Steven Rostedt wrote: > On Thu, 21 Apr 2022 17:27:40 +0100 > Mark Rutland <mark.rutland@arm.com> wrote: > > > We can initialize the ops pointer to a default ops that does the whole > > __do_for_each_ftrace_ops() dance. > > OK, I think I understand now. What you are doing is instead of creating a > trampoline that has all the information in the trampoline, you add nops to > all the functions where you can place the information in the nops (before > the function), and then have the trampoline just read that information to > find the ops pointer as well as the function to call.
FWIW, I had a go at mocking that up:
https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/ftrace/per-callsite-ops
Aside from some bodges required to ensure the patch site is suitably aligned (which I think can be cleaned up somewhat), I don't think it looks that bad.
I wasn't sure how exactly to wire that up in the core code, so all the patch sites are initialized with a default ops that calls arch_ftrace_ops_list_func(), but it looks like it should be possible to wire that up in the core with some refactoring.
> I guess you could have two trampolines as well. One that always calls the > list loop, and one that calls the data stored in front of the function that > was just called the trampoline. As it is always safe to call the loop > function, you could have the call call that trampoline first, set up the > specific data before the function, then call the trampoline that will read > it.
I was thinking we could just patch the ops with a default ops that called the list loop, as my patches default them to.
> And same thing for tear down.
I wasn't sure how teardown was meant to work in general. When we want to remove an ops structure, or a trampoline, how do we ensure those are no longer in use before we remove them? I can see how we can synchronize the updates to the kernel text, but I couldn't spot how we handle a thread being in the middle of a trampoline.
Thanks, Mark.
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