A Time Consuming Pitfall for 32-Bit Applications on AArch64

Running 32-bit legacy applications on 64-bit AArch64 Linux systems can lead to performance issues if not configured correctly. Unlike x86_64, AArch64 requires separate GCC toolchains for 32-bit and 64-bit applications. When building the Linux kernel, enabling the CONFIG_COMPAT option is necessary, but it must be paired with the CROSS_COMPILE_COMPAT variable pointing to a 32-bit toolchain. If this is overlooked, the kernel will not provide a virtual dynamic shared object (vDSO) for 32-bit applications, resulting in slower performance due to context switches for system calls like gettimeofday(2). A benchmark demonstrated that querying the current time was over 25 times slower without the vDSO. The author encountered this issue while working on a project using Yocto and had to patch the kernel build process to incorporate CROSS_COMPILE_COMPAT support. To check for vDSO availability, Linux provides methods such as using the auxiliary vector AT_SYSINFO_EHDR or the glibc function getauxval(3). It is crucial for developers to ensure their build systems support the necessary configurations to avoid performance pitfalls when running legacy applications.

- AArch64 requires separate GCC toolchains for 32-bit and 64-bit applications.

- Failure to configure CROSS_COMPILE_COMPAT can lead to significant performance degradation.

- The vDSO allows faster system calls by avoiding context switches.

- Benchmark tests showed a dramatic performance difference with and without vDSO.

- Developers should verify vDSO availability using Linux tools and ensure build systems support necessary configurations.