From: "Arnd Bergmann" <arnd@arndb.de>
To: "David Laight" <david.laight.linux@gmail.com>
Cc: "Ian Rogers" <irogers@google.com>,
"Marc Kleine-Budde" <mkl@pengutronix.de>,
"Thomas Gleixner" <tglx@linutronix.de>,
"Stefan Kerkmann" <s.kerkmann@pengutronix.de>,
"James E . J . Bottomley" <James.Bottomley@hansenpartnership.com>,
"Helge Deller" <deller@gmx.de>,
"Andy Lutomirski" <luto@kernel.org>,
"Vincenzo Frascino" <vincenzo.frascino@arm.com>,
"Arnaldo Carvalho de Melo" <acme@redhat.com>,
linux-parisc@vger.kernel.org, linux-kernel@vger.kernel.org,
"Eric Biggers" <ebiggers@google.com>,
"Alexander Viro" <viro@zeniv.linux.org.uk>,
"Christophe Leroy" <christophe.leroy@csgroup.eu>,
"Jason A . Donenfeld" <Jason@zx2c4.com>
Subject: Re: [Regression] [PATCH v5 2/4] vdso: Switch get/put unaligned from packed struct to memcpy
Date: Tue, 20 Oct 2026 10:41:27 +0200 [thread overview]
Message-ID: <100aecfe-f3f6-4058-9f3b-1eb942f23253@app.fastmail.com> (raw)
In-Reply-To: <20261020093604.286e072a@pumpkin>
On Tue, Oct 20, 2026, at 10:36, David Laight wrote:
>
> Historically it has definitely done that.
> And you can't 'cast away' the alignment implicit in a type.
> The alignment can get tracked into an inlined function.
> Plenty of crashes on sparc32 from that one.
>
> Any structure that might be misaligned in memory better be packed.
>
> But what is actually wrong with using byte access? eg:
> u32 get_le32(u8 *p) { return p[0] | p[1] << 8 | p[2] << 16 | p[3] << 24; }
> On x86, arm64, ppc (etc) both gcc and clang generate a single memory read
> (byteswapped if necessary).
>
> https://godbolt.org/z/d33hvesr8
IIRC this is one of the versions we had in the past on some architectures,
but the packed struct version ended up being better in some cases and
never worse. It is possible that on new compilers are just as good with this
version, and if someone can show that being true I think we can do
that here as well.
Arnd
next prev parent reply other threads:[~2026-10-20 8:42 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-10-16 20:51 [PATCH v5 0/4] Switch get/put unaligned to use memcpy Ian Rogers
2025-10-16 20:51 ` [PATCH v5 1/4] parisc: Inline a type punning version of get_unaligned_le32 Ian Rogers
2026-01-13 13:47 ` [tip: timers/vdso] parisc: Inline a type punning version of get_unaligned_le32() tip-bot2 for Ian Rogers
2026-01-14 8:01 ` tip-bot2 for Ian Rogers
2025-10-16 20:51 ` [PATCH v5 2/4] vdso: Switch get/put unaligned from packed struct to memcpy Ian Rogers
2025-10-19 17:24 ` David Laight
2026-01-13 13:47 ` [tip: timers/vdso] vdso: Switch get/put_unaligned() from packed struct to memcpy() tip-bot2 for Ian Rogers
2026-01-14 8:01 ` tip-bot2 for Ian Rogers
2026-09-28 15:39 ` [Regression] [PATCH v5 2/4] vdso: Switch get/put unaligned from packed struct to memcpy Stefan Kerkmann
2026-09-28 15:58 ` Ian Rogers
2026-10-06 12:31 ` Marc Kleine-Budde
2026-10-07 10:59 ` Arnd Bergmann
2026-10-09 21:28 ` Ian Rogers
2026-10-10 10:26 ` Christophe Leroy (CS GROUP)
2026-10-10 12:20 ` Ian Rogers
2026-10-10 14:06 ` Arnd Bergmann
2026-10-10 15:04 ` Ian Rogers
2026-10-19 12:37 ` Marc Kleine-Budde
2026-10-19 15:10 ` Ian Rogers
2026-10-19 16:27 ` Arnd Bergmann
2026-10-19 17:04 ` Ian Rogers
2026-10-19 21:00 ` Arnd Bergmann
2026-10-19 21:14 ` Ian Rogers
2026-10-19 22:05 ` Arnd Bergmann
2026-10-20 16:21 ` Ian Rogers
2026-10-20 19:27 ` David Laight
2026-10-20 8:36 ` David Laight
2026-10-20 8:41 ` Arnd Bergmann [this message]
2026-10-20 10:01 ` David Laight
2025-10-16 20:51 ` [PATCH v5 3/4] tools headers: Update the linux/unaligned.h copy with the kernel sources Ian Rogers
2026-01-13 13:47 ` [tip: timers/vdso] " tip-bot2 for Ian Rogers
2026-01-14 8:01 ` tip-bot2 for Ian Rogers
2025-10-16 20:51 ` [PATCH v5 4/4] tools headers: Remove unneeded ignoring of warnings in unaligned.h Ian Rogers
2026-01-13 13:47 ` [tip: timers/vdso] " tip-bot2 for Ian Rogers
2026-01-14 8:01 ` tip-bot2 for Ian Rogers
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