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    SubjectRe: BPF vs objtool again
    On Wed, Apr 29, 2020 at 04:51:59PM -0500, Josh Poimboeuf wrote:
    > On Thu, Jul 18, 2019 at 12:14:08PM -0700, tip-bot for Josh Poimboeuf wrote:
    > > Commit-ID: 3193c0836f203a91bef96d88c64cccf0be090d9c
    > > Gitweb: https://git.kernel.org/tip/3193c0836f203a91bef96d88c64cccf0be090d9c
    > > Author: Josh Poimboeuf <jpoimboe@redhat.com>
    > > AuthorDate: Wed, 17 Jul 2019 20:36:45 -0500
    > > Committer: Thomas Gleixner <tglx@linutronix.de>
    > > CommitDate: Thu, 18 Jul 2019 21:01:06 +0200
    > >
    > > bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()
    >
    > For some reason, this
    >
    > __attribute__((optimize("-fno-gcse")))
    >
    > is disabling frame pointers in ___bpf_prog_run(). If you compile with
    > CONFIG_FRAME_POINTER it'll show something like:
    >
    > kernel/bpf/core.o: warning: objtool: ___bpf_prog_run.cold()+0x7: call without frame pointer save/setup

    you mean it started to disable frame pointers from some version of gcc?
    It wasn't doing this before, since objtool wasn't complaining, right?
    Sounds like gcc bug?

    > Also, since GCC 9.1, the GCC docs say "The optimize attribute should be
    > used for debugging purposes only. It is not suitable in production
    > code." That doesn't sound too promising.
    >
    > So it seems like this commit should be reverted. But then we're back to
    > objtool being broken again in the RETPOLINE=n case, which means no ORC
    > coverage in this function. (See above commit for the details)
    >
    > Some ideas:
    >
    > - Skip objtool checking of that func/file (at least for RETPOLINE=n) --
    > but then it won't have ORC coverage.
    >
    > - Get rid of the "double goto" in ___bpf_prog_run(), which simplifies it
    > enough for objtool to understand -- but then the text explodes for
    > RETPOLINE=y.

    How that will look like?
    That could be the best option.

    > - Add -fno-gfcse to the Makefile for kernel/bpf/core.c -- but then that
    > affects the optimization of other functions in the file. However I
    > don't think the impact is significant.
    >
    > - Move ___bpf_prog_run() to its own file with the -fno-gfcse flag. I'm
    > thinking this could be the least bad option. Alexei?

    I think it would be easier to move some of the hot path
    functions out of core.c instead.
    Like *ksym*, BPF_CALL*, bpf_jit*, bpf_prog*.
    I think resulting churn will be less.
    imo it's more important to keep git blame history for interpreter
    than for the other funcs.
    Sounds like it's a fix that needs to be sent for the next RC ?
    Please send a patch for bpf tree then.

    Daniel, thoughts?

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