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Wed, 7 Oct 2026 06:59:57 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-ThreadId: AICZ2hJ9SF6o Date: Wed, 07 Oct 2026 12:59:16 +0200 From: "Arnd Bergmann" To: "Marc Kleine-Budde" , "Stefan Kerkmann" Cc: "Ian Rogers" , "James E . J . Bottomley" , "Helge Deller" , "Andy Lutomirski" , "Thomas Gleixner" , "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: <2f6094ff-f585-4b8b-8c25-20c11d1bfc7d@app.fastmail.com> In-Reply-To: <20261006-dynamic-succinct-woodpecker-a36c7f-mkl@pengutronix.de> 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> 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 6, 2026, at 14:31, Marc Kleine-Budde wrote: > Cc+ Arnd Thanks for the Cc! > On 28.09.2026 17:39:15, Stefan Kerkmann wrote: >> Hi Ian, >> >> On 10/16/25 22:51, Ian Rogers wrote: >> > Type punning is necessary for get/put unaligned but the use of a >> > packed struct violates strict aliasing rules, requiring >> > -fno-strict-aliasing to be passed to the C compiler. Switch to using >> > memcpy so that -fno-strict-aliasing isn't necessary. >> > >> > Signed-off-by: Ian Rogers >> > --- > >> Is this an accepted trade-off? My understanding is that the kernel is always >> built with -fno-strict-aliasing, so the packed-struct type punning was well >> defined there, and the __packed annotation is what lets GCC generate valid >> code for the unaligned access. The previous upstream version was the result of endless discussions, and it looks like changing it to the memcpy version was premature. At the time we unified all architectures to use a common implentation, this was the only one that resulted in correct and fast code on all architectures, so I don't understand why this was just applied without including everyone who was involved in coming up with the version that was replaced. My feeling is that we should just revert this. I'm not sure about the motivation for the change. It sounds like this was meant to be used in userland code, and that clashed with assumptions we make in the kernel, but I don't think that is sufficient reason for regressing kernel code. Arnd