On Jun 28, 2019, at 12:41 PM, Yu-cheng Yu [off-list ref] wrote:

The previous discussion of the IBT legacy code bitmap is here:

   https://lkml.org/lkml/2019/6/6/1032

When CET Indirect Branch Tracking (IBT) is enabled, the processor expects
every branch target is an ENDBR instruction, or the target's address is
marked as legacy in the legacy code bitmap.  The bitmap covers the whole
user-mode address space (TASK_SIZE_MAX for 64-bit, TASK_SIZE for IA32),
and each bit represents one page of linear address range.

This patch introduces VM_IBT for the bitmap.

There’s no need to allocate a bit for this and to clutter up the fault code with special cases. Use _install_special_mapping(), please.  If you need to make it more flexible to cover your use case, please do so.