This comment is perhaps misplaced. Should it go somewhere where the 'trap frame CFI information' is applied?

ast should be handled for kthreads trampolines as well

FRED event entry points need the annotations

Shouldn't .cfi_sections directive specified once, at the beginning of the asm source? At least, this is my impression from reading the gas documentation for it.

I do not see this macro as useful, it only obfuscates the reading. And the passing of '.' as 'name' is confusing.

Did you checked this both with clang as and binutils as? I remember I having troubles with non-gprs, in particular segment registers names. See sys/amd64/amd64/sigtramp.S. If some #ifdef 0's can be enabled there, I think it is quite important for userspace debugging.

Might be call this macro 'trap_cfi_trapframe'?

      .text
      .globl  cfi_segments
      .type   cfi_segments, @function
cfi_segments:
      .cfi_startproc simple
      .cfi_def_cfa    %rsp, 40
      .cfi_offset     %cs, -32
      .cfi_offset     49, -24 /* rflags */
      .cfi_offset     %ss, -8
      ret
      .cfi_endproc
      .size   cfi_segments, . - cfi_segments

      .section .note.GNU-stack,"",@progbits

Both clang 21 and as 2.44 compile this code and emit same result (through llvm-dwarfdump --eh-frame). Both compiles to fail for %rflags instead of 49.

I have not tested on older versions though.

Do you want trap_cfi_entry state=full to be state=trapframe as well?

Do we need cfi info in .eh_frame? It is loaded into the memory at runtime then. AFAIR there is no consumer of that data in kernel. Since it is for benefit of the offline debugger, can it be .debug_frame instead?

And I think that the .cfi_sections directive should go into the .S file directly. It is very surprising to have include file adjust code generation.

Perhaps. Also the default value for the macro arg does not need to be repeated at the use site.

clang drops .cfi_signal_frame for .cfi_sections .dwarf_frame. I don't know if this is intended though.

On top of that, having .eh_frame in stripped kernel still gives correct backtrace when debug symbol file and full kernel are unavailable.

clang drops .cfi_signal_frame for .cfi_sections .dwarf_frame. I don't know if this is intended though. On top of that, having .eh_frame in stripped kernel still gives correct backtrace when debug symbol file and full kernel are unavailable.

At least apic_vector.S and atpic_vector.S are not self-contained asm compilation units. They are not compiled alone, but included into exception.S.

I believe the old test for TDP_KTHREAD was somewhat more correct. Most of the code uses TDP_KTHREAD to detect kernel threads. I am not even sure that the other use of P_KPROC in copy_thread() is entirely correct, we are copying the thread state there.

Do we compile kernel with async exception tables? I remember that we do not, and then standard exception tables are not enough (because we want to unwind across interrupt frames), am I right?

We don't for amd64, but explicit cfi directives still emit FDE in the compiled kernel. The only reason I used .cfi_signal_frame here is to use raw PC.

nop                         /* terminal CFI */
trap_cfi_trapframe
cmpb    $0, fred            /* trapframe CFI */

If exception happens at cmpb and the interrupt handler isn't marked as signal frame, it will calculate pc by subtracting 1 from return address and choose terminal frame. We want trapframe CFI here so unwinder shouldn't subtract 1 from saved PC.