Are extended instruction sets (SSE, MMX) used in Linux kernel?

Question

Well, they bring (should bring at least) great increase in performance, isn’t it?

So, I haven’t seen any Linux kernel sources, but ‘d love to ask: are they used somehow? (In this case – there must be some special “code-cap” for system that has no such instructions?)

Answer 1

The SSE and MMX instruction sets have limited value outside of audio/video and gaming work. You might find a few explicit uses in dark corners of the kernel, but I wouldn't count on it. The answer in the general case is "no, they are not used", nor are they used in most non-kernel/userspace applications.

The kernel does sometimes optionally use certain x86 instructions that are specific to certain CPUs (e.g. present on some AMD or Intel models but not all, nor vice-versa), such as syscall, but these are different from the SIMD instruction sets you're referring to, and are not part of some wider set of similarly-themed extensions.

After Mark's answer, I went looking. The only place I could easily identify them being used is in the RAID 6 library (which also has support for AltiVec, which is the PowerPC SIMD instruction set).

(Be wary just grepping the tree, there are a lot of spots where the kernel "knows" about SSE/MMX to support user-space applications, but isn't actually using it. Also a couple cases of unfortunate variable names that have absolutely nothing to do with SSE, e.g. in the SCTP implementation.)

Answer 2

There are severe restrictions on using vector registers and floating point registers in kernel code. See e.g. chapter 6.3 of "Calling conventions for different C++ compilers and operating systems". http://www.agner.org/optimize/#manuals

Answer 3

They are used in the kernel for a few things, such as

• Software RAID
• Encryption (possibly)

However, I believe it always checks their presence first.