[RFC PATCH 13/20] x86/entry: Stop excluding the entry code from KCOV

[Sasha Levin <sashal@kernel.org>]

The x86 entry code has been excluded from KCOV since commit 20355e5f73a7
("x86/entry: Exclude low level entry code from sanitizing"), when
noinstr couldn't keep KCOV out of noinstr functions.

Since commit 540540d06e9d ("kcov: add __no_sanitize_coverage to fix
noinstr for all architectures"), noinstr implies __no_sanitize_coverage,
and on x86 objtool removes any KCOV call left in noinstr code. KCOV
instruments the code after inlining, so __always_inline helpers don't
carry it into noinstr functions either. What remains instrumentable in
arch/x86/entry/ is the system call dispatch and the ptrace system call
hooks, which run in task context.

Drop the exclusion, so that noinstr alone keeps KCOV out of the x86
entry code, like it does for the other coverage implementations. A
traced read(-1) now reports 18 instead of 12 PCs, the additional ones
being the system call dispatch.

The generic entry code has to stay excluded, but for a different reason
than its Makefile gives: parts of the interrupt exit code, such as
irqentry_exit_cond_resched(), run after leaving hardirq context, where
KCOV adds their coverage to the trace of the interrupted task. Without
the exclusion, they show up in the traces of system calls interrupted
by the timer. Document that instead.

Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/x86/entry/Makefile | 1 -
 kernel/entry/Makefile   | 3 +++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/Makefile b/arch/x86/entry/Makefile
index 83b4762d6ecba..cd24c28358f97 100644
--- a/arch/x86/entry/Makefile
+++ b/arch/x86/entry/Makefile
@@ -5,7 +5,6 @@
 
 KASAN_SANITIZE := n
 UBSAN_SANITIZE := n
-KCOV_INSTRUMENT := n
 
 CFLAGS_REMOVE_syscall_32.o	= $(CC_FLAGS_FTRACE)
 CFLAGS_REMOVE_syscall_64.o	= $(CC_FLAGS_FTRACE)
diff --git a/kernel/entry/Makefile b/kernel/entry/Makefile
index f220bae86b123..2c837b7513c71 100644
--- a/kernel/entry/Makefile
+++ b/kernel/entry/Makefile
@@ -4,6 +4,9 @@
 # as long as these things cannot be disabled per function.
 KASAN_SANITIZE := n
 UBSAN_SANITIZE := n
+
+# Parts of the interrupt exit code run after leaving hardirq context, where KCOV
+# would add their coverage to the trace of the interrupted task.
 KCOV_INSTRUMENT := n
 
 # Branch profiling isn't noinstr-safe
-- 
2.53.0