From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
"H. Peter Anvin" <hpa@zytor.com>,
Song Liu <songliubraving@fb.com>,
Thomas Gleixner <tglx@linutronix.de>,
Steven Rostedt <rostedt@goodmis.org>,
Kairui Song <kasong@redhat.com>,
Daniel Borkmann <daniel@iogearbox.net>,
Alexei Starovoitov <ast@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
linux-tip-commits@vger.kernel.org
Subject: Re: [tip:x86/urgent] bpf: Fix ORC unwinding in non-JIT BPF code
Date: Mon, 8 Jul 2019 15:15:37 -0700 [thread overview]
Message-ID: <CAADnVQJqT8o=_6P6xHjwxrXqX9ToSb0cTfoOcm2Xcha3KRNNSw@mail.gmail.com> (raw)
In-Reply-To: <20190707055209.xqyopsnxfurhrkxw@treble>
On Sat, Jul 6, 2019 at 10:52 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> On Sat, Jul 06, 2019 at 08:32:06PM -0500, Josh Poimboeuf wrote:
> > On Sat, Jul 06, 2019 at 10:29:42PM +0200, Ingo Molnar wrote:
> > > Hm, I get this new build warning on x86-64 defconfig-ish kernels plus
> > > these enabled:
> > >
> > > CONFIG_BPF=y
> > > CONFIG_BPF_JIT=y
> > >
> > > kernel/bpf/core.o: warning: objtool: ___bpf_prog_run()+0x8da: sibling call from callable instruction with modified stack frame
> >
> > I assume you have CONFIG_RETPOLINE disabled? For some reason that
> > causes GCC to add 166 indirect jumps to that function, which is giving
> > objtool trouble. Looking into it.
>
> Alexei, do you have any objections to setting -fno-gcse for
> ___bpf_prog_run()? Either for the function or the file? Doing so seems
> to be recommended by the GCC manual for computed gotos. It would also
> "fix" one of the issues. More details below.
>
> Details:
>
> With CONFIG_RETPOLINE=n, there are a couple of GCC optimizations in
> ___bpf_prog_run() which objtool is having trouble with.
>
> 1)
>
> The function has:
>
> select_insn:
> goto *jumptable[insn->code];
>
> And then a bunch of "goto select_insn" statements.
>
> GCC is basically replacing
>
> select_insn:
> jmp *jumptable(,%rax,8)
> ...
> ALU64_ADD_X:
> ...
> jmp select_insn
> ALU_ADD_X:
> ...
> jmp select_insn
>
> with
>
> select_insn:
> jmp *jumptable(, %rax, 8)
> ...
> ALU64_ADD_X:
> ...
> jmp *jumptable(, %rax, 8)
> ALU_ADD_X:
> ...
> jmp *jumptable(, %rax, 8)
>
>
> It does that 166 times.
>
> For some reason, it doesn't do the optimization with retpolines
> enabled.
>
> Objtool has never seen multiple indirect jump sites which use the same
> jump table. This is relatively trivial to fix (I already have a
> working patch).
>
> 2)
>
> After doing the first optimization, GCC then does another one which is
> a little trickier. It replaces:
>
> select_insn:
> jmp *jumptable(, %rax, 8)
> ...
> ALU64_ADD_X:
> ...
> jmp *jumptable(, %rax, 8)
> ALU_ADD_X:
> ...
> jmp *jumptable(, %rax, 8)
>
> with
>
> select_insn:
> mov jumptable, %r12
> jmp *(%r12, %rax, 8)
> ...
> ALU64_ADD_X:
> ...
> jmp *(%r12, %rax, 8)
> ALU_ADD_X:
> ...
> jmp *(%r12, %rax, 8)
>
> The problem is that it only moves the jumptable address into %r12
> once, for the entire function, then it goes through multiple recursive
> indirect jumps which rely on that %r12 value. But objtool isn't yet
> smart enough to be able to track the value across multiple recursive
> indirect jumps through the jump table.
>
> After some digging I found that the quick and easy fix is to disable
> -fgcse. In fact, this seems to be recommended by the GCC manual, for
> code like this:
>
> -fgcse
> Perform a global common subexpression elimination pass. This
> pass also performs global constant and copy propagation.
>
> Note: When compiling a program using computed gotos, a GCC
> extension, you may get better run-time performance if you
> disable the global common subexpression elimination pass by
> adding -fno-gcse to the command line.
>
> Enabled at levels -O2, -O3, -Os.
>
> This code indeed relies extensively on computed gotos. I don't know
> *why* disabling this optimization would improve performance. In fact
> I really don't see how it could make much of a difference either way.
>
> Anyway, using -fno-gcse makes optimization #2 go away and makes
> objtool happy, with only a fix for #1 needed.
>
> If -fno-gcse isn't an option, we might be able to fix objtool by using
> the "first_jump_src" thing which Peter added, improving it such that
> it also takes table jumps into account.
Sorry for delay. I'm mostly offgrid until next week.
As far as -fno-gcse.. I don't mind as long as it doesn't hurt performance.
Which I suspect it will :(
All these indirect gotos are there for performance.
Single indirect goto and a bunch of jmp select_insn
are way slower, since there is only one instruction
for cpu branch predictor to work with.
When every insn is followed by "jmp *jumptable"
there is more room for cpu to speculate.
It's been long time, but when I wrote it the difference
between all indirect goto vs single indirect goto was almost 2x.
next prev parent reply other threads:[~2019-07-08 22:15 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-28 1:50 [PATCH v4 0/2] x86: bpf unwinder fixes Josh Poimboeuf
2019-06-28 1:50 ` [PATCH v4 1/2] objtool: Add support for C jump tables Josh Poimboeuf
2019-06-29 5:58 ` [tip:x86/urgent] " tip-bot for Josh Poimboeuf
2019-07-09 12:01 ` [tip:x86/debug] " tip-bot for Josh Poimboeuf
2019-06-28 1:50 ` [PATCH v4 2/2] bpf: Fix ORC unwinding in non-JIT BPF code Josh Poimboeuf
2019-06-28 15:37 ` Alexei Starovoitov
2019-06-29 5:58 ` [tip:x86/urgent] " tip-bot for Josh Poimboeuf
2019-07-06 20:29 ` Ingo Molnar
2019-07-07 1:32 ` Josh Poimboeuf
2019-07-07 5:52 ` Josh Poimboeuf
2019-07-08 22:15 ` Alexei Starovoitov [this message]
2019-07-08 22:38 ` Josh Poimboeuf
2019-07-08 22:49 ` Alexei Starovoitov
2019-07-08 22:53 ` Josh Poimboeuf
2019-07-08 23:02 ` Josh Poimboeuf
2019-07-08 23:16 ` Alexei Starovoitov
2019-07-09 17:47 ` Josh Poimboeuf
2019-07-09 18:02 ` Alexei Starovoitov
2019-07-09 19:17 ` Josh Poimboeuf
2019-07-09 19:26 ` Daniel Borkmann
2019-07-09 12:02 ` [tip:x86/debug] " tip-bot for Josh Poimboeuf
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