x86/mm: Prevent early boot triple-faults with instrumentation

x86/mm: Prevent early boot triple-faults with instrumentation

Commit in Fixes added a global TLB flush on the early boot path, after
the kernel switches off of the trampoline page table.

Compiler profiling options enabled with GCOV_PROFILE add additional
measurement code on clang which needs to be initialized prior to
use. The global flush in x86_64_start_kernel() happens before those
initializations can happen, leading to accessing invalid memory.
GCOV_PROFILE builds with gcc are still ok so this is clang-specific.

The second issue this fixes is with KASAN: for a similar reason,
kasan_early_init() needs to have happened before KASAN-instrumented
functions are called.

Therefore, reorder the flush to happen after the KASAN early init
and prevent the compilers from adding profiling instrumentation to
native_write_cr4().

Fixes: f154f290855b ("x86/mm/64: Flush global TLB on boot and AP bringup")
Reported-by: "J. Bruce Fields" <bfields@fieldses.org>
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Tested-by: Carel Si <beibei.si@intel.com>
Tested-by: "J. Bruce Fields" <bfields@fieldses.org>
Link: https://lore.kernel.org/r/20211209144141.GC25654@xsang-OptiPlex-9020