Merge branch 'x86-rep-insns': x86 user copy clarifications

Merge branch 'x86-rep-insns': x86 user copy clarifications

Merge my x86 user copy updates branch.

This cleans up a lot of our x86 memory copy code, particularly for user
accesses.  I've been pushing for microarchitectural support for good
memory copying and clearing for a long while, and it's been visible in
how the kernel has aggressively used 'rep movs' and 'rep stos' whenever
possible.

And that micro-architectural support has been improving over the years,
to the point where on modern CPU's the best option for a memory copy
that would become a function call (as opposed to being something that
can just be turned into individual 'mov' instructions) is now to inline
the string instruction sequence instead.

However, that only makes sense when we have the modern markers for this:
the x86 FSRM and FSRS capabilities ("Fast Short REP MOVS/STOS").

So this cleans up a lot of our historical code, gets rid of the legacy
marker use ("REP_GOOD" and "ERMS") from the memcpy/memset cases, and
replaces it with that modern reality.  Note that REP_GOOD and ERMS end
up still being used by the known large cases (ie page copyin gand
clearing).

The reason much of this ends up being about user memory accesses is that
the normal in-kernel cases are done by the compiler (__builtin_memcpy()
and __builtin_memset()) and getting to the point where we can use our
instruction rewriting to inline those to be string instructions will
need some compiler support.

In contrast, the user accessor functions are all entirely controlled by
the kernel code, so we can change those arbitrarily.

Thanks to Borislav Petkov for feedback on the series, and Jens testing
some of this on micro-architectures I didn't personally have access to.

* x86-rep-insns:
  x86: rewrite '__copy_user_nocache' function
  x86: remove 'zerorest' argument from __copy_user_nocache()
  x86: set FSRS automatically on AMD CPUs that have FSRM
  x86: improve on the non-rep 'copy_user' function
  x86: improve on the non-rep 'clear_user' function
  x86: inline the 'rep movs' in user copies for the FSRM case
  x86: move stac/clac from user copy routines into callers
  x86: don't use REP_GOOD or ERMS for user memory clearing
  x86: don't use REP_GOOD or ERMS for user memory copies
  x86: don't use REP_GOOD or ERMS for small memory clearing
  x86: don't use REP_GOOD or ERMS for small memory copies