Re: [PATCH v4 11/11] perf auxtrace: Add compat_auxtrace_mmap__{read_head|write_tail}

From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Leo Yan <leo.yan@linaro.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Mathieu Poirier <mathieu.poirier@linaro.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Mike Leach <mike.leach@linaro.org>,
	linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
	coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v4 11/11] perf auxtrace: Add compat_auxtrace_mmap__{read_head|write_tail}
Date: Mon, 12 Jul 2021 15:44:11 +0100	[thread overview]
Message-ID: <20210712144410.GE22278@shell.armlinux.org.uk> (raw)
In-Reply-To: <20210711104105.505728-12-leo.yan@linaro.org>

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.

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