linux-kernel - Re: [PATCH v4 11/11] perf auxtrace: Add compat_auxtrace_mmap__{read_head|write

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Date:   Mon, 12 Jul 2021 15:44:11 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mike Leach <mike.leach@...aro.org>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 11/11] perf auxtrace: Add
 compat_auxtrace_mmap__{read_head|write_tail}

On Sun, Jul 11, 2021 at 06:41:05PM +0800, Leo Yan wrote:
> When perf runs in compat mode (kernel in 64-bit mode and the perf is in
> 32-bit mode), the 64-bit value atomicity in the user space cannot be
> assured, E.g. on some architectures, the 64-bit value accessing is split
> into two instructions, one is for the low 32-bit word accessing and
> another is for the high 32-bit word.

Does this apply to 32-bit ARM code on aarch64? I would not have thought
it would, as the structure member is a __u64 and
compat_auxtrace_mmap__read_head() doesn't seem to be marking anything
as packed, so the compiler _should_ be able to use a LDRD instruction
to load the value.

Is this a problem noticed on non-ARM architectures?

Thanks.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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