I ported the x86 approach to allowing SIMD in kernel mode, even in an interrupt, to powerpc[1], but I couldn't help wonder if it would be possible without disabling preemption. The scheduler just has to check the per-cpu in_kernel_fpu, and if it is set, save the fpu state of the kernel thread, similar to what is done for user-space threads with use_mode(). This way WireGuard would be fast even on preempt-rt kernels (it currently turns off SIMD on them). Other crypto in the kernel is woorse, and just unconditionally turns off preempt for each block of data (look at arch/x86/crypto/chacha_glue.c ). Note that kernel_fpu_begin() disables preemption, but I don't believe it has to. Is there any complications with this approach that I am missing? -Shawn Landden [1] https://lists.ozlabs.org/pipermail/linuxppc-dev/2019-May/190704.html
