Re: ERROR: modpost: "kmsan_handle_dma" [drivers/virtio/virtio_ring.ko] undefined!

From: Josh Poimboeuf
Date: Tue Feb 18 2025 - 20:17:42 EST


On Tue, Feb 18, 2025 at 08:47:26AM -0800, Josh Poimboeuf wrote:
> On Tue, Feb 18, 2025 at 02:28:55PM +0100, Peter Zijlstra wrote:
> > On Tue, Feb 18, 2025 at 12:48:57PM +0100, Sebastian Andrzej Siewior wrote:
> > > On 2025-02-15 06:42:36 [+0800], kernel test robot wrote:
> > > > >> arch/x86/kvm/cpuid.o: warning: objtool: do_cpuid_func+0x2428: undefined stack state
> > >
> >
> > > From the assembly it seems to make sense:
> > > | 110ae: 49 89 e0 mov %rsp,%r8
> > > stash for later
> > > | 110b1: 48 85 db test %rbx,%rbx
> > > | 110b4: c7 00 00 00 00 00 movl $0x0,(%rax)
> > > | 110ba: 45 89 7e 14 mov %r15d,0x14(%r14)
> > > | 110be: 0f 85 40 01 00 00 jne 11204 <do_cpuid_func+0x22f4>
> > > …
> > > | 11204: 44 8b 74 24 38 mov 0x38(%rsp),%r14d
> > > | 11209: 44 89 f7 mov %r14d,%edi
> > > | 1120c: 4d 89 c7 mov %r8,%r15

[ Adding Masahiro for #1 ]

1)

Masahiro, one problem we've seen is that "make LLVM=1
arch/x86/kvm/cpuid.s" doesn't match the final compiled .o binary. Is it
a problem with cmd_cc_s_c? The original .config is here:

https://lore.kernel.org/202502150634.qjxwSeJR-lkp@xxxxxxxxx

If I manually run the original gcc command with -S, cpuid.s looks right.

2)

Peter, Sebastian, the objtool warning is caused by Clang interpreting
ASM_CALL_CONSTRAINT a little too literally. It's stashing the stack
pointer and restoring it later to comply with the "+r" rsp constraint.
Which makes perfect sense, but is also 100% unexpected.

I "fixed" it with the below, but it's not a proper fix as
ASM_CALL_CONSTRAINT is still needed for CONFIG_UNWINDER_FRAME_POINTER.

So I think the options are:

- Make objtool smarter

- Improve ASM_CALL_CONSTRAINT somehow. /me wonders if "memory"
clobber has the same effect?

- Drop support for x86-64 frame pointers altogether, along with
ASM_CALL_CONSTRAINT. Is there any reason to keep frame pointers
around anymore for 64 bit? AFAICT objtool is 100% mandatory for all
configs, and ORC has proven to be solid.

diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
index fea56b04f436..7414f0c7cc0e 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -394,7 +394,7 @@ int paravirt_disable_iospace(void);
PVOP_TEST_NULL(op); \
asm volatile(ALTERNATIVE(PARAVIRT_CALL, ALT_CALL_INSTR, \
ALT_CALL_ALWAYS) \
- : call_clbr, ASM_CALL_CONSTRAINT \
+ : call_clbr \
: paravirt_ptr(op), \
##__VA_ARGS__ \
: "memory", "cc" extra_clbr); \
@@ -409,7 +409,7 @@ int paravirt_disable_iospace(void);
asm volatile(ALTERNATIVE_2(PARAVIRT_CALL, \
ALT_CALL_INSTR, ALT_CALL_ALWAYS, \
alt, cond) \
- : call_clbr, ASM_CALL_CONSTRAINT \
+ : call_clbr \
: paravirt_ptr(op), \
##__VA_ARGS__ \
: "memory", "cc" extra_clbr); \

--
Josh