On Mon, Sep 27, 2021 at 07:33:40PM +0200, Alexander Lochmann wrote:

The existing trace mode stores PCs in execution order. This could lead
to a buffer overflow if sufficient amonut of kernel code is executed.
Thus, a user might not see all executed PCs. KCOV_MODE_UNIQUE favors
completeness over execution order. While ignoring the execution order,
it marks a PC as exectued by setting a bit representing that PC. Each
bit in the shared buffer represents every fourth byte of the text
segment.  Since a call instruction on every supported architecture is
at least four bytes, it is safe to just store every fourth byte of the
text segment.

I'm still trying to wake up, but why are call instruction more important
than other instructions? Specifically, I'd think any branch instruction
matters for coverage.

More specifically, x86 can do a tail call with just 2 bytes.