Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE_RETPOLINE*

[Josh Poimboeuf]

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@xxxxxxxxxxxxx> 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.

-- 
Josh