Re: plugins: Missing Store Exclusive Memory Accesses

[Aaron Lindsay via <qemu-devel@nongnu.org>]

On Sep 17 12:05, Alex Bennée wrote:
> Aaron Lindsay <aaron@os.amperecomputing.com> writes:
> > I recently noticed that the plugin interface does not appear to be
> > emitting callbacks to functions registered via
> > `qemu_plugin_register_vcpu_mem_cb` for AArch64 store exclusives. This
> > would include instructions like `stxp  w16, x2, x3, [x4]` (encoding:
> > 0xc8300c82). Seeing as how I'm only running with a single CPU, I don't
> > see how this could be due to losing exclusivity after the preceding
> > `ldxp`.
> 
> The exclusive handling is a bit special due to the need to emulate it's
> behaviour using cmpxchg primitives.
> 
> >
> > In looking at QEMU's source, I *think* this is because the
> > `gen_store_exclusive` function in translate-a64.c is not making the same
> > calls to `plugin_gen_mem_callbacks` & company that are being made by
> > "normal" stores handled by functions like `tcg_gen_qemu_st_i64` (at
> > least in my case; I do see some code paths under `gen_store_exclusive`
> > call down into `tcg_gen_qemu_st_i64` eventually, but it appears not all
> > of them do?).
> 
> The key TCG operation is the cmpxchg which does the effective store. For
> -smp 1 we should use normal ld and st tcg ops. For > 1 it eventually
> falls to tcg_gen_atomic_cmpxchg_XX which is a helper. That eventually
> ends up at:
> 
>   atomic_trace_rmw_post
> 
> which should be where things are hooked.

When I open this up in gdb, I see that I'm getting the following call
graph for the `stxp` instruction in question (for -smp 1):

gen_store_exclusive -> gen_helper_paired_cmpxchg64_le

In other words, I'm taking the `s->be_data == MO_LE` else/if clause.

I do not see where the helper behind that (defined in helper-a64.c as
`uint64_t HELPER(paired_cmpxchg64_le)...`) is calling in to generate
plugin callbacks in this case. Am I missing something?

-Aaron