[RFC][PATCH 6/6] x86/mce: Dont use noinstr for now

[Peter Zijlstra]

We call do_machine_check() from a 'safe' context, instrumentation
should be possible. However, due to the nature of the beast, MCE is
fragile and we really rather would not want to hit additional code.

Currently this is done by marking do_machine_check() as noinstr,
however there's at least one objtool warning because of that:

  vmlinux.o: warning: objtool: do_machine_check()+0x650: call to queue_task_work() leaves .noinstr.text section

trying to fix that leads down a rabbit-hole. A _lot_ of code would end
up having to be marked noinstr outside of the MCE code. While
possible, doing so is undesired because it would hide it from
instrumentation for all other users.

Instead of using noinstr, kill instrumentation for all of mce/. This
switches MCE over to a best-effort but non-validated mode. Doing
better will require a significant investment of time and effort.

The obvious problem with tracing the MCE is that the tracer will
trigger a further MCE and we DIE an instant death.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/kernel/cpu/mce/Makefile |    7 +++++++
 arch/x86/kernel/cpu/mce/core.c   |    6 +-----
 2 files changed, 8 insertions(+), 5 deletions(-)

--- a/arch/x86/kernel/cpu/mce/Makefile
+++ b/arch/x86/kernel/cpu/mce/Makefile
@@ -1,4 +1,11 @@
 # SPDX-License-Identifier: GPL-2.0
+
+# No instrumentation for MCE code
+KASAN_SANITIZE := n
+UBSAN_SANITIZE := n
+KCOV_INSTRUMENT := n
+ccflags-remove-$(CONFIG_FUNCTION_TRACER) += $(CC_FLAGS_FTRACE)
+
 obj-y				=  core.o severity.o genpool.o
 
 obj-$(CONFIG_X86_ANCIENT_MCE)	+= winchip.o p5.o
--- a/arch/x86/kernel/cpu/mce/core.c
+++ b/arch/x86/kernel/cpu/mce/core.c
@@ -1299,7 +1299,7 @@ static void queue_task_work(struct mce *
  * backing the user stack, tracing that reads the user stack will cause
  * potentially infinite recursion.
  */
-noinstr void do_machine_check(struct pt_regs *regs)
+void do_machine_check(struct pt_regs *regs)
 {
 	DECLARE_BITMAP(valid_banks, MAX_NR_BANKS);
 	DECLARE_BITMAP(toclear, MAX_NR_BANKS);
@@ -1987,10 +1987,6 @@ static __always_inline void exc_machine_
 		return;
 
 	irq_state = irqentry_nmi_enter(regs);
-	/*
-	 * The call targets are marked noinstr, but objtool can't figure
-	 * that out because it's an indirect call. Annotate it.
-	 */
 	instrumentation_begin();
 
 	machine_check_vector(regs);