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([2a01:e0a:982:cbb0:fbee:23cf:fdf3:bad4]) by smtp.gmail.com with ESMTPSA id n16-20020a5d4850000000b002c6e8cb612fsm7354785wrs.92.2023.02.27.08.38.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 27 Feb 2023 08:38:50 -0800 (PST) Message-ID: <33b58877-5167-c453-e686-1d10cdca66c0@linaro.org> Date: Mon, 27 Feb 2023 17:38:49 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.2 From: Neil Armstrong Subject: Re: [PATCH v1 0/3] Meson A1 32-bit support To: Arnd Bergmann , Dmitry Rokosov Cc: Alexey Romanov , Rob Herring , krzysztof.kozlowski+dt@linaro.org, Kevin Hilman , jbrunet@baylibre.com, martin.blumenstingl@googlemail.com, Linus Walleij , devicetree@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-amlogic@lists.infradead.org, linux-kernel@vger.kernel.org, kernel@sberdevices.ru References: <20230222115020.55867-1-avromanov@sberdevices.ru> <8e5f9bfa-d612-cd43-d722-d04c40938c62@linaro.org> <20230227142809.kujmrraf3pcdhqyn@CAB-WSD-L081021> <20230227155100.hhl4yvkyfqfyoa6h@CAB-WSD-L081021> Content-Language: en-US Organization: Linaro Developer Services In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20230227_083855_147588_53A7DA44 X-CRM114-Status: GOOD ( 18.93 ) X-BeenThere: linux-amlogic@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: neil.armstrong@linaro.org Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-amlogic" Errors-To: linux-amlogic-bounces+linux-amlogic=archiver.kernel.org@lists.infradead.org On 27/02/2023 17:15, Arnd Bergmann wrote: > On Mon, Feb 27, 2023, at 16:51, Dmitry Rokosov wrote: >> On Mon, Feb 27, 2023 at 03:58:50PM +0100, Arnd Bergmann wrote: >>> >>> I would argue that is a problem with buildroot, and using a 32-bit >>> kernel is not something we should encourage over fixing buildroot >>> to do it right, or building the kernel separately from the rootfs. >>> >>> We do allow building support for a couple of ARMv8 SoCs in 32-bit >>> mode, but that is usually because they ship with a 32-bit bootrom >>> and cannot actually run a 64-bit kernel. >> >> To be honest, I didn't know about this principle. It looks like a very >> rational approach "start from max supported bitness". >> Based on overall maintainers opinion, we have to prepare a patch for >> buildroot to support compat mode :) > > That would be great, thanks a lot! > > For what it's worth, the main arguments in favor of running a 64-bit > kernel with compat user space over a 32-bit kernel are support for: > > - larger RAM sizes without highmem (most 32-bit kernels only > support 768MB of lowmem, and highmem sucks) > - larger virtual address space (4GB vs 3GB or less) > - CPU specific errata workarounds (arch/arm/ only has those for 32-bit cpus) > - mitigations for common attacks such as spectre > - security hardening that depends on larger address space > (KASLR, BTI, ptrauth, PAN, ...) > - emulating instructions that were removed in Armv8 (setend, swp, ...) > > Most of these don't apply in userspace, so the incentive to > run smaller 32-bit userland on systems with less than 1GB of > RAM usually outweighs the benefits of 64-bit userspace. Thanks for the details! Neil > > Arnd _______________________________________________ linux-amlogic mailing list linux-amlogic@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-amlogic