The latest proposal for kernel AMX support (updated today) is here: https://lore.kernel.org/lkml/20210630060226.24652-1-chang.seok.bae@xxxxxxxxx/ The main challenge for AMX is not context switch performance. Hardware recognizes INIT state (the common case) and skips that data transfer when it is not needed. The main challenge for AMX is compatibility. Specifically, user signal stack growth. The legacy ABI is that we put an uncompacted XSTATE image on the signal stack. In the default stack case, this isn't a problem, but when a user allocates an alternative signal stack, the 8K of XSTATE growth that AMX can exceed what the user allocated. The new system call tells the kernel that the application can handle it. (it can do this by not using altsigstack, or by using the updated stack size advertised by glibc 2.34 and later, or some other means)
