linux-kernel - Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE

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Message-ID: <CAADnVQ+iMysKSKBGzx7Wa+ygpr9nTJbRo4eGYADLFDE4PmtjOQ@mail.gmail.com>
Date:   Thu, 21 Oct 2021 16:42:12 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>, Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE_RETPOLINE*

On Thu, Oct 21, 2021 at 4:38 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> On Thu, Oct 21, 2021 at 04:24:33PM -0700, Alexei Starovoitov wrote:
> > On Thu, Oct 21, 2021 at 3:40 PM Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > On Thu, Oct 21, 2021 at 11:03:33AM -0700, Alexei Starovoitov wrote:
> > >
> > > > > I nicked it from emit_bpf_tail_call() in the 32bit jit :-) It seemed a
> > > > > lot more robust than the 64bit one and I couldn't figure out why the
> > > > > difference.
> > > >
> > > > Interesting. Daniel will recognize that trick then :)
> > >
> > > > > Is there concurrency on the jit?
> > > >
> > > > The JIT of different progs can happen in parallel.
> > >
> > > In that case I don't think the patch is safe. I'll see if I can find a
> > > variant that doesn't use static storage.
> >
> > The variable can only change from one fixed value to another fixed value.
> > Different threads will compute the same value. So I think it's safe
> > as-is. READ_ONCE/WRITE_ONCE won't hurt though.
>
> But the size of the generated code differs based on the
> emit_bpf_tail_call_indirect() args: 'callee_regs_used' and
> 'stack_depth'.  So the fixed value can change.

Ahh. Right. It's potentially a different offset for every prog.
Let's put it into struct jit_context then.