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Tue, 20 Oct 2026 04:42:23 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 X-ThreadId: AICZ2hJ9SF6o Date: Tue, 20 Oct 2026 10:41:27 +0200 From: "Arnd Bergmann" To: "David Laight" Cc: "Ian Rogers" , "Marc Kleine-Budde" , "Thomas Gleixner" , "Stefan Kerkmann" , "James E . J . Bottomley" , "Helge Deller" , "Andy Lutomirski" , "Vincenzo Frascino" , "Arnaldo Carvalho de Melo" , linux-parisc@vger.kernel.org, linux-kernel@vger.kernel.org, "Eric Biggers" , "Alexander Viro" , "Christophe Leroy" , "Jason A . Donenfeld" Message-Id: <100aecfe-f3f6-4058-9f3b-1eb942f23253@app.fastmail.com> In-Reply-To: <20261020093604.286e072a@pumpkin> References: <20251016205126.2882625-1-irogers@google.com> <20251016205126.2882625-3-irogers@google.com> <47f8c30b-e326-4ff0-a2bc-5683209a9e2d@pengutronix.de> <20261006-dynamic-succinct-woodpecker-a36c7f-mkl@pengutronix.de> <2f6094ff-f585-4b8b-8c25-20c11d1bfc7d@app.fastmail.com> <20261019-rapid-loutish-porpoise-ac4efb-mkl@pengutronix.de> <18c92ae1-deaa-4c09-adca-157819cf005b@app.fastmail.com> <20261020093604.286e072a@pumpkin> Subject: Re: [Regression] [PATCH v5 2/4] vdso: Switch get/put unaligned from packed struct to memcpy Content-Type: text/plain Content-Transfer-Encoding: 7bit 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