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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e258424bcsm913222f8f.55.2026.10.20.14.20.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Oct 2026 14:20:10 -0700 (PDT) Date: Tue, 20 Oct 2026 22:20:09 +0100 From: David Laight To: Ian Rogers Cc: Arnd Bergmann , 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" Subject: Re: [Regression] [PATCH v5 2/4] vdso: Switch get/put unaligned from packed struct to memcpy Message-ID: <20261020222009.54f6d35a@pumpkin> In-Reply-To: <20261020202744.55c52a49@pumpkin> References: <20251016205126.2882625-1-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> <20261020202744.55c52a49@pumpkin> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) 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 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Tue, 20 Oct 2026 20:27:44 +0100 David Laight 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