带有AArch64和32位应用程序的Android内核

Question

How can Android device run AARCH64 kernel while running multiple(possibly several dozens) of A32 apk processes? Does it mean the kernel will constantly make millions of execution state switches?

Or Android kernel is not going to be AARCH64 in the near future?

Answer 1

The kernel is making those "millions of execution state switches" anyway regardless; every time it takes an interrupt or system call, or schedules a new task, it's preserving an EL0 context on the switch to EL1, then restoring either the same or a different context on return. From that position the difference between preserving/restoring a 32-bit EL0 vs. a 64-bit one isn't much more than the number of registers involved. The specific AArch32/AArch64 detail is simply part of the processor mode in the SPSR, dealt with by the hardware as an inherent part of the exception entry/return mechanism.

The bulk of the compatibility layer in the arm64 kernel is actually the translation between the 32-bit and 64-bit system call ABIs; the rest just kind of falls out of the way the architecture was designed.

As for "the near future", well, Android L with its full 64-bit support has been out for a while already...