Re: [PATCH] objtool: Support Clang RAX DRAP sequence

[Peter Zijlstra]

On Mon, Mar 16, 2026 at 05:47:56PM -0700, Josh Poimboeuf wrote:
> Recent Clang can use RAX as a temporary register for the DRAP stack
> alignment sequence.  Add support for that.
> 
> Fixes the following warning:
> 
>   vmlinux.o: error: objtool: vmw_host_printf+0xd: unknown CFA base reg 0
> 
> Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
> Closes: https://lore.kernel.org/cefefdd1-7b82-406d-8ff4-e4b167e45ee6@xxxxxxxxxxxxxxxx
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/orc_types.h       | 1 +
>  arch/x86/kernel/unwind_orc.c           | 8 ++++++++
>  tools/arch/x86/include/asm/orc_types.h | 1 +
>  tools/objtool/arch/x86/decode.c        | 3 +++
>  tools/objtool/arch/x86/orc.c           | 5 +++++
>  5 files changed, 18 insertions(+)
> 
> diff --git a/arch/x86/include/asm/orc_types.h b/arch/x86/include/asm/orc_types.h
> index e0125afa53fb..b3cc7970fa54 100644
> --- a/arch/x86/include/asm/orc_types.h
> +++ b/arch/x86/include/asm/orc_types.h
> @@ -37,6 +37,7 @@
>  #define ORC_REG_R13			7
>  #define ORC_REG_BP_INDIRECT		8
>  #define ORC_REG_SP_INDIRECT		9
> +#define ORC_REG_AX			10
>  #define ORC_REG_MAX			15

I'm 'annoyed' at the placement of that register. I know its not
important, but I can't silence my OCD saying that AX should come before
DX.

Also, SP comes before BP and both before DI.

Something like so perhaps?

---
diff --git a/arch/x86/include/asm/orc_types.h b/arch/x86/include/asm/orc_types.h
index e0125afa53fb..5df398761c70 100644
--- a/arch/x86/include/asm/orc_types.h
+++ b/arch/x86/include/asm/orc_types.h
@@ -28,15 +28,19 @@
  * and GCC realigned stacks.
  */
 #define ORC_REG_UNDEFINED		0
-#define ORC_REG_PREV_SP			1
+
+#define ORC_REG_AX			1
 #define ORC_REG_DX			2
-#define ORC_REG_DI			3
+#define ORC_REG_SP			3
 #define ORC_REG_BP			4
-#define ORC_REG_SP			5
+#define ORC_REG_DI			5
 #define ORC_REG_R10			6
 #define ORC_REG_R13			7
-#define ORC_REG_BP_INDIRECT		8
+
+#define ORC_REG_PREV_SP			8
 #define ORC_REG_SP_INDIRECT		9
+#define ORC_REG_BP_INDIRECT		10
+
 #define ORC_REG_MAX			15
 
 #define ORC_TYPE_UNDEFINED		0