From: David Laight <david.laight.linux@gmail.com>
To: Ian Rogers <irogers@google.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
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 22:20:09 +0100 [thread overview]
Message-ID: <20261020222009.54f6d35a@pumpkin> (raw)
In-Reply-To: <20261020202744.55c52a49@pumpkin>
On Tue, 20 Oct 2026 20:27:44 +0100
David Laight <david.laight.linux@gmail.com> wrote:
> I suspect it happens less often with __packed.
> You can probably add volatile to stop the other merging.
>
> It really might be necessary to give up on the C versions and use asm()
> (at least for the unaligned access case).
This probably works for unaligned 64bit - and is much better than the code
the compiler generates:
unsigned long get_u64_le(void *s)
{
unsigned long p = (unsigned long)s;
unsigned int shift = 8 * (p & 7);
return *(unsigned long *)(p & ~7) >> shift |
*(unsigned long *)((p + 7) & ~7) << (-shift & 63);
}
For native BE the two shifts need swapping.
Not worth it for 32bit unless the alignment is known.
(And neither gcc nor clang do that for misaligned members in aligned struct.)
>
> David
next prev parent reply other threads:[~2026-10-20 21:20 UTC|newest]
Thread overview: 36+ 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 21:20 ` David Laight [this message]
2026-10-20 8:36 ` David Laight
2026-10-20 8:41 ` Arnd Bergmann
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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