On 05.12.23 17:29, Julien Grall wrote:
Hi Arnd,On 05/12/2023 14:37, Arnd Bergmann wrote:On Tue, Dec 5, 2023, at 15:19, Julien Grall wrote:On 05/12/2023 14:10, Arnd Bergmann wrote:On Tue, Dec 5, 2023, at 15:01, Julien Grall wrote:On 05/12/2023 13:59, Jan Beulich wrote:On 05.12.2023 14:46, Julien Grall wrote:This would repeat the mistake that we had in Linux in the past (and still do in some drivers): Simply dereferencing a misaligned pointer is always a bug, even on architectures that have efficient unaligned load/store instructions, because C makes this undefined behavior and gcc has optimizations that assume e.g. 'int *' to never have the lower two bits set [1].Just to clarify, I haven't suggested to use 'int *'. My point was more that I don't think that the helpers would work as-is on arm32 because even if the ISA allows a direct access, we are setting the bit in SCTLR to disable unaligned access. As Juergen is proposing a common header, then I could ask him to do the work to confirm that the helpers properly work on arm32. But I think this is unfair.When I introduced the helpers in Linux, I showed that these produce the best output on all modern compilers (at least gcc-5, probably earlier) for both architectures that allow unaligned access and for those that don't. We used to have architecture specific helpers depending on what each architecture could do, but all the other variants we had were either wrong or less efficient. If for some reason an Arm system is configured to trap all unaligned access, then you must already pass -mno-unaligned-access to the compiler to prevent certain optimizations, and then the helpers will still behave correctly (the same way they do on armv5, which never has unaligned access). On armv7 with -munaligned-access, the same functions only prevent the use of stm/ldm and strd/ldrd but still use ldr/str.Unfortunately we don't explicitely do. This would explain why I saw some issues with certain compiler [1].So I agree that adding -mno-unaligned-access for arm32 makes sense. @Juergen, do you want me to send a patch?
Yes, will do. Juergen
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