On Tue, Jun 23, 2020 at 06:51:22AM -0400, Bryan Steele wrote: > On Mon, Jun 22, 2020 at 11:10:10PM -0700, jo...@armadilloaerospace.com wrote: > > Are SSE instructions allowed in the AMD64 kernel? Is #ifdef __SSE__ > > a sufficient guard? > > > > I have a rasops32 putchar with SSE that is 2x faster. > > No, in general you cannot using FP instructions in the kernel, also the > kernel is often compiled with -msoft-float on platforms that support it.
More specifically using the FP instructions the kernel incurs a performance penalty from having to save and restore the FP registers. There are some helper functions for that in the very limited cases where it's done, see fpu_kernel_enter/exit() in arch/amd64/amd64/fpu.c. But these are i386/amd64-only.
