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Cameron , Guenter Roeck , Jens Reidel Subject: Re: [PATCH] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN Message-ID: References: <20250919184025.15416-1-will@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable In-Reply-To: X-Provags-ID: V03:K1:ynWIxZ1tNUOtNq3OpSo1gVYFL39JkXSxaiKX8oqOFqkEVgtpz8J 9qNFZMZwwKaLfnPzISwqX7SOCwRnvVq7/RTq5mA0hTQdVxjvtTVvcTCQ3hBsFarzVbwBHHX bZPewIyvfR1/jNJAF9YAgeuYhXfMszO9H/kMAX6GMjgZttOcO3+CL/CNgnHeJ5VC5QYWQzE VitCpmWnvJpXJ8fg71/5A== UI-OutboundReport: notjunk:1;M01:P0:18xmb1yJEfk=;DTU36Y/5P/nHrDyN1sYEg89YYrG psIBFfUF0ALoI8q5C6tEUDenvSwut7N37g/4bW1+AAO8JkfAUSKvILeBeosW9bwpdIdB7pn0s t29DiHjE/Xol5N3uin39LWIrmhRn4+oZzGlBtDu4rWMobF1brCzcLdqjoGkDYgSzU38wqDUHm fCVtJg4vPoFylHY2b/tfh6K8dQaK6zl/drtTX+fMTPpTyX7DPRIPmsLVbSAnxFhYkyO1bGm8u ZQQdLsLMlEQ83aoES36j46W+ZfKmPdtFy8dNRVWvWGtIrisaZyIsSoNyDSrhUGzq4HzkvRYEs 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TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250929_131652_525264_825E7BB4 X-CRM114-Status: GOOD ( 26.04 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, Sep 29, 2025 at 11:52:48AM +0100, Catalin Marinas wrote: > On Mon, Sep 29, 2025 at 02:18:55AM +0200, J. Neusch=C3=A4fer wrote: > > On Fri, Sep 19, 2025 at 07:40:25PM +0100, Will Deacon wrote: > > > Big-endian arm64 configurations are vanishingly rare, yet we still c= laim > > > to support them in Linux despite very limited testing or visible > > > interest. Supporting big-endian adds unnecessary burden to reviewers= and > > > contributors which, without any known active users, is hard to justi= fy. > > > For example, recent work to improve our futex routines and to implem= ent > > > nested virtualisation support is non-trivially complicated by having= to > > > support both big- and little-endianness. > > >=20 > > > Back in 2019 [1], it was claimed that Huawei were using arm64 big-en= dian > > > machines in their telecommunication products but I don't know whethe= r > > > that's still the case and certainly haven't seen any patch contribut= ions > > > to help support or maintain it. > > >=20 > > > Make CPU_BIG_ENDIAN depend on BROKEN as an initial deprecation step > > > towards its removal. > >=20 > > As of this month (September 2025) there's a new community project [1,2= ] > > to revive aarch64_be. Jens Reidel (CC'd) and I are involved in it. We'= ve > > been fixing several aarch64_be-related bugs, but mostly[3] in userspac= e, > > because the kernel support is pretty solid. > >=20 > > So, just so it doesn't go unmentioned, there is interest in keeping > > big-endian ARM64 alive. >=20 > It's nice to see it gets testing but is there any actual beyond you and > Jens (i.e. in production somewhere like Huawei's case)? We want to > assess whether it's worth the kernel maintenance and testing hassle and > for how long. We don't have any commercial or large deployments, no. It's currently one build host and manual testing on additional machines. I'd be sad to see big-endian arm64 support removed because it's an opportunity to test for endian-related bugs (in various packages, not just the kernel) on widely available and reasonably fast hardware. I understand the maintenance cost argument, and I'm fine with the CONFIG_BROKEN dependency, even if it might be applied to features that cause problems in the future (such as nested virt) =E2=80=94 it's easy to = work around, and features can be fixed as needed. A complete removal will likely be harder to revert, though, especially once code simplifications are made based on supporting only little-endian. That's my concern. Best regards, J. Neusch=C3=A4fer