yes. see Documentation/HOWTO (3rd paragraph of the introduction) or the kernel-hacking document that is compiled from Documentation/DocBook/kernel-hacking.tmpl (chapter 3):
what you probably saw were save areas or the FPU emulator? there is some code using MMX registers, but these are working on relatively large datasets (thus the MMX optimization) and you can afford the overhead for saving/restoring the status and watching for context switches. for single variables this is nonsense, choose a suitable unit (nanoseconds, lightyears) and use integers.
yes
yes. see Documentation/HOWTO (3rd paragraph of the introduction) or the kernel-hacking document that is compiled from Documentation/DocBook/kernel-hacking.tmpl (chapter 3):
http://kernel.org/doc/htmldocs/kernel-hacking.html
what you probably saw were save areas or the FPU emulator? there is some code using MMX registers, but these are working on relatively large datasets (thus the MMX optimization) and you can afford the overhead for saving/restoring the status and watching for context switches. for single variables this is nonsense, choose a suitable unit (nanoseconds, lightyears) and use integers.