x86/nospec: Fix evaluate_nospec() code generation under Clang

x86/nospec: Fix evaluate_nospec() code generation under Clang

It turns out that evaluate_nospec() code generation is not safe under Clang.
Given:

  void eval_nospec_test(int x)
  {
      if ( evaluate_nospec(x) )
          asm volatile ("nop #true" ::: "memory");
      else
          asm volatile ("nop #false" ::: "memory");
  }

Clang emits:

  <eval_nospec_test>:
         0f ae e8                lfence
         85 ff                   test   %edi,%edi
         74 02                   je     <eval_nospec_test+0x9>
         90                      nop
         c3                      ret
         90                      nop
         c3                      ret

which is not safe because the lfence has been hoisted above the conditional
jump.  Clang concludes that both barrier_nospec_true()'s have identical side
effects and can safely be merged.

Clang can be persuaded that the side effects are different if there are
different comments in the asm blocks.  This is fragile, but no more fragile
that other aspects of this construct.

Introduce barrier_nospec_false() with a separate internal comment to prevent
Clang merging it with barrier_nospec_true() despite the otherwise-identical
content.  The generated code now becomes:

  <eval_nospec_test>:
         85 ff                   test   %edi,%edi
         74 05                   je     <eval_nospec_test+0x9>
         0f ae e8                lfence
         90                      nop
         c3                      ret
         0f ae e8                lfence
         90                      nop
         c3                      ret

which has the correct number of lfence's, and in the correct place.

Link: https://github.com/llvm/llvm-project/issues/55084
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>