From: "Alex Bennée" <alex.bennee@linaro.org>
To: Aaron Lindsay <aaron@os.amperecomputing.com>
Cc: cota@braap.org, richard.henderson@linaro.org, qemu-devel@nongnu.org
Subject: Re: plugins: Missing Store Exclusive Memory Accesses
Date: Fri, 17 Sep 2021 12:05:09 +0100 [thread overview]
Message-ID: <87fsu3tppe.fsf@linaro.org> (raw)
In-Reply-To: <YUOssEF1lND+Rhsr@strawberry.localdomain>
Aaron Lindsay <aaron@os.amperecomputing.com> writes:
> Hello,
>
> 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.
> Does my initial guess check out? And, if so, does anyone have insight
> into how to fix this issue most cleanly/generically? I suspect if/when I
> debug my particular case I can discover one code path to fix, but I'm
> wondering if my discovery may be part of a larger class of cases which
> fell through the cracks and ought to be fixed together.
Have you got simple example of a test case?
>
> Thanks for any help,
>
> Aaron
--
Alex Bennée
next prev parent reply other threads:[~2021-09-17 11:21 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-16 20:44 plugins: Missing Store Exclusive Memory Accesses Aaron Lindsay
2021-09-17 11:05 ` Alex Bennée [this message]
2021-09-17 14:44 ` Aaron Lindsay via
2021-09-21 20:28 ` Aaron Lindsay via
2021-09-22 20:22 ` Aaron Lindsay via
2021-10-20 17:12 ` Aaron Lindsay via
2021-10-20 17:54 ` Alex Bennée
2021-10-20 20:49 ` Aaron Lindsay via
2021-10-21 12:28 ` Alex Bennée
2021-10-21 20:40 ` Aaron Lindsay via
2021-10-22 8:37 ` Alex Bennée
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