x86: Introduce barrier_nospec

x86: Introduce barrier_nospec

Rename the open coded form of this instruction sequence from
rdtsc_ordered() into a generic barrier primitive, barrier_nospec().

One of the mitigations for Spectre variant1 vulnerabilities is to fence
speculative execution after successfully validating a bounds check. I.e.
force the result of a bounds check to resolve in the instruction pipeline
to ensure speculative execution honors that result before potentially
operating on out-of-bounds data.

No functional changes.

Suggested-by: Linus Torvalds <[email protected]>
Suggested-by: Andi Kleen <[email protected]>
Suggested-by: Ingo Molnar <[email protected]>
Signed-off-by: Dan Williams <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: [email protected]
Cc: Tom Lendacky <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: Al Viro <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/151727415361.33451.9049453007262764675.stgit@dwillia2-desk3.amr.corp.intel.com