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    SubjectRe: [tip:x86/urgent] bpf: Fix ORC unwinding in non-JIT BPF code
    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.

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