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Date:   Tue, 19 Jul 2022 10:34:08 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Andrew Cooper <Andrew.Cooper3@citrix.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Johannes Wikner <kwikner@ethz.ch>,
        Alyssa Milburn <alyssa.milburn@linux.intel.com>,
        Jann Horn <jannh@google.com>, "H.J. Lu" <hjl.tools@gmail.com>,
        Joao Moreira <joao.moreira@intel.com>,
        Joseph Nuzman <joseph.nuzman@intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>
Subject: Re: [patch 37/38] x86/bpf: Emit call depth accounting if required
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On Mon, Jul 18, 2022 at 10:30:01PM -0700, Alexei Starovoitov wrote:
> On Sat, Jul 16, 2022 at 4:18 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> > @@ -1431,19 +1437,26 @@ st:                     if (is_imm8(insn->off))
> >                         break;
> >
> >                         /* call */
> > -               case BPF_JMP | BPF_CALL:
> > +               case BPF_JMP | BPF_CALL: {
> > +                       int offs;
> > +
> >                         func = (u8 *) __bpf_call_base + imm32;
> >                         if (tail_call_reachable) {
> >                                 /* mov rax, qword ptr [rbp - rounded_stack_depth - 8] */
> >                                 EMIT3_off32(0x48, 0x8B, 0x85,
> >                                             -round_up(bpf_prog->aux->stack_depth, 8) - 8);
> > -                               if (!imm32 || emit_call(&prog, func, image + addrs[i - 1] + 7))
> > +                               if (!imm32)
> >                                         return -EINVAL;
> > +                               offs = 7 + x86_call_depth_emit_accounting(&prog, func);
> 
> It's a bit hard to read all the macro magic in patches 28-30,
> but I suspect the asm inside
> callthunk_desc.template
> that will be emitted here before the call
> will do
> some math on %rax
> movq %rax, PER_CPU_VAR(__x86_call_depth).
> 
> Only %rax register is scratched by the callthunk_desc, right?
> If so, it's ok for all cases except this one.
> See the comment few lines above
> after if (tail_call_reachable)
> and commit ebf7d1f508a7 ("bpf, x64: rework pro/epilogue and tailcall
> handling in JIT")
> We use %rax to keep the tail_call count.
> The callthunk_desc would need to preserve %rax.
> I guess extra push %rax/pop %rax would do it.

The accounting template is basically:

	sarq $5, PER_CPU_VAR(__x86_call_depth)

No registeres used (with debugging on it's a few more memops).