x86/paravirt: Work around GCC inlining bugs when compiling paravirt ops

x86/paravirt: Work around GCC inlining bugs when compiling paravirt ops

As described in:

  77b0bf55bc67: ("kbuild/Makefile: Prepare for using macros in inline assembly code to work around asm() related GCC inlining bugs")

GCC's inlining heuristics are broken with common asm() patterns used in
kernel code, resulting in the effective disabling of inlining.

The workaround is to set an assembly macro and call it from the inline
assembly block. As a result GCC considers the inline assembly block as
a single instruction. (Which it isn't, but that's the best we can get.)

In this patch we wrap the paravirt call section tricks in a macro,
to hide it from GCC.

The effect of the patch is a more aggressive inlining, which also
causes a size increase of kernel.

      text     data     bss      dec     hex  filename
  18147336 10226688 2957312 31331336 1de1408  ./vmlinux before
  18162555 10226288 2957312 31346155 1de4deb  ./vmlinux after (+14819)

The number of static text symbols (non-inlined functions) goes down:

  Before: 40053
  After:  39942 (-111)

[ mingo: Rewrote the changelog. ]

Tested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alok Kataria <akataria@vmware.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: virtualization@lists.linux-foundation.org
Link: http://lkml.kernel.org/r/20181003213100.189959-8-namit@vmware.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>