linux-kernel - Re: BPF vs objtool again

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Date:   Wed, 29 Apr 2020 19:13:00 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     x86@...nel.org, tglx@...utronix.de, linux-kernel@...r.kernel.org,
        mingo@...nel.org, hpa@...or.com, ast@...nel.org,
        peterz@...radead.org, rdunlap@...radead.org,
        Arnd Bergmann <arnd@...db.de>, bpf@...r.kernel.org,
        daniel@...earbox.net
Subject: Re: BPF vs objtool again

On Wed, Apr 29, 2020 at 04:41:59PM -0700, Alexei Starovoitov wrote:
> 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@...hat.com>
> > > AuthorDate: Wed, 17 Jul 2019 20:36:45 -0500
> > > Committer:  Thomas Gleixner <tglx@...utronix.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?

I actually think this warning has been around for a while.  I just only
recently looked at it.  I don't think anything changed in GCC, it's just
that almost nobody uses CONFIG_FRAME_POINTER, so it wasn't really
noticed.

> > 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.

For example:

#define GOTO    ({ goto *jumptable[insn->code]; })

and then replace all 'goto select_insn' with 'GOTO;'

The problem is that with RETPOLINE=y, the function text size grows from
5k to 7k, because for each of the 160+ retpoline JMPs, GCC (stupidly)
reloads the jump table register into a scratch register.

> > - 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.

I can make a patch, what file would you recommend moving those hot path
functions to?

-- 
Josh