Re: [PATCH] arm64/mm: Fix annotated branch unbootable kernel

On Wed, Dec 31, 2025 at 04:44:05AM -0800, Breno Leitao wrote:

The arm64 kernel doesn't boot with annotated branches
(PROFILE_ANNOTATED_BRANCHES) enabled and CONFIG_DEBUG_VIRTUAL together.

Bisecting it, I found that disabling branch profiling in arch/arm64/mm
solved the problem. Narrowing down a bit further, I found that
physaddr.c is the file that needs to have branch profiling disabled to
get the machine to boot.

I suspect that it might invoke some ftrace helper very early in the boot
process and ftrace is still not enabled(!?).

Disable branch profiling for physaddr.o to allow booting an arm64
machine with CONFIG_PROFILE_ANNOTATED_BRANCHES and
CONFIG_DEBUG_VIRTUAL together.

Cc: stable@vger.kernel.org
Fixes: ec6d06efb0bac ("arm64: Add support for CONFIG_DEBUG_VIRTUAL")
Signed-off-by: Breno Leitao <leitao@debian.org>
---
Another approach is to disable profiling on all arch/arm64 code, similarly to
x86, where DISABLE_BRANCH_PROFILING is called for all arch/x86 code. See
commit 2cbb20b008dba ("tracing: Disable branch profiling in noinstr
code").

Yes, let's start with arch/arm64/. We know that's safe and then if
somebody wants to make it finer-grained, it's on them to figure out a
way to do it without playing whack-a-mole.

Will