Re: [tip:x86/alternatives 14/14] arch/x86/kvm/kvm.o: warning: objtool: .altinstr_replacement+0xc5: call without frame pointer save/setup

Peter Zijlstra
2024-06-18T10:47:08Z

On Tue, Jun 18, 2024 at 12:42:35PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 14, 2024 at 05:22:28PM +0200, Borislav Petkov wrote:
> > On Fri, Jun 14, 2024 at 04:15:14PM +0800, kernel test robot wrote:
> > > tree: https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git x86/alternatives
> > > head: f776e41fdcc4141876ef6f297318ab04c2382eb7
> > > commit: f776e41fdcc4141876ef6f297318ab04c2382eb7 [14/14] x86/alternative: Replace the old macros
> > > config: x86_64-buildonly-randconfig-r001-20211209 (https://download.01.org/0day-ci/archive/20240614/202406141648.jO9qNGLa-lkp@intel.com/config)
> > > compiler: gcc-13 (Ubuntu 13.2.0-4ubuntu3) 13.2.0
> > > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240614/202406141648.jO9qNGLa-lkp@intel.com/reproduce)
> > >
> > > If you fix the issue in a separate patch/commit (i.e. not just a new version of
> > > the same patch/commit), kindly add following tags
> > > | Reported-by: kernel test robot <lkp@intel.com>
> > > | Closes: https://lore.kernel.org/oe-kbuild-all/202406141648.jO9qNGLa-lkp@intel.com/
> > >
> > > All warnings (new ones prefixed by >>):
> > >
> > > >> arch/x86/kvm/kvm.o: warning: objtool: .altinstr_replacement+0xc5: call without frame pointer save/setup
> >
> > That thing comes from:
> >
> > SYM_FUNC_START(vmx_do_interrupt_irqoff)
> > VMX_DO_EVENT_IRQOFF CALL_NOSPEC _ASM_ARG1
> > SYM_FUNC_END(vmx_do_interrupt_irqoff)
> >
> > at the end of arch/x86/kvm/vmx/vmenter.S.
>
> You sure? Afaict the thing is:
>
> $ OBJTOOL_ARGS="--verbose" make O=tmp-build/ arch/x86/kvm/
> ...
> arch/x86/kvm/kvm.o: warning: objtool: .altinstr_replacement+0xc5: call without frame pointer save/setup
> arch/x86/kvm/kvm.o: warning: objtool: em_loop.part.0+0x29: (alt)
> arch/x86/kvm/kvm.o: warning: objtool: em_loop.part.0+0x0: <=== (sym)
> 0000 0000000000028220 <em_loop.part.0>:
> 0000 28220: 0f b6 47 61 movzbl 0x61(%rdi),%eax
> 0004 28224: 3c e2 cmp $0xe2,%al
> 0006 28226: 74 2c je 28254 <em_loop.part.0+0x34>
> 0008 28228: 48 8b 57 10 mov 0x10(%rdi),%rdx
> 000c 2822c: 83 f0 05 xor $0x5,%eax
> 000f 2822f: 48 c1 e0 04 shl $0x4,%rax
> 0013 28233: 25 f0 00 00 00 and $0xf0,%eax
> 0018 28238: 81 e2 d5 08 00 00 and $0x8d5,%edx
> 001e 2823e: 80 ce 02 or $0x2,%dh
> 0021 28241: 48 05 00 00 00 00 add $0x0,%rax 28243: R_X86_64_32S em_setcc
> 0027 28247: 52 push %rdx
> 0028 28248: 9d popf
> 0029 28249: ff d0 call *%rax
> 002b 2824b: 90 nop
> 002c 2824c: 90 nop
> 002d 2824d: 90 nop
> 002e 2824e: 31 d2 xor %edx,%edx
> 0030 28250: 84 c0 test %al,%al
> 0032 28252: 74 1d je 28271 <em_loop.part.0+0x51>
> 0034 28254: 55 push %rbp
> 0035 28255: 48 63 b7 d0 00 00 00 movslq 0xd0(%rdi),%rsi
> 003c 2825c: 48 03 b7 90 00 00 00 add 0x90(%rdi),%rsi
> 0043 28263: 48 89 e5 mov %rsp,%rbp
> 0046 28266: e8 05 fe ff ff call 28070 <assign_eip>
> 004b 2826b: 5d pop %rbp
> 004c 2826c: e9 00 00 00 00 jmp 28271 <em_loop.part.0+0x51> 2826d: R_X86_64_PLT32 __x86_return_thunk-0x4
> 0051 28271: 89 d0 mov %edx,%eax
> 0053 28273: e9 00 00 00 00 jmp 28278 <em_loop.part.0+0x58> 28274: R_X86_64_PLT32 __x86_return_thunk-0x4
> 0058 28278: 0f 1f 84 00 00 00 00 00 nopl 0x0(%rax,%rax,1)
>
> Which is in fact test_cc() from em_loop().
>
> Now, that .config is a FRAME_POINTER build, and the fast-call thing
> don't do frames, so it is right to complain. Big question is why it
> didn't complain previously I suppose.

ha! the below (whitespace damanged because copy/paste sucks), also works.

For some reason GCC thought it was fun to put that CALL outside the callframe, silly thing.

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 5d4c86133453..c8cc578646d0 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -1069,7 +1069,7 @@ static __always_inline u8 test_cc(unsigned int condition, unsigned long flags)

flags = (flags & EFLAGS_MASK) | X86_EFLAGS_IF;
asm("push %[flags]; popf; " CALL_NOSPEC
- : "=a"(rc) : [thunk_target]"r"(fop), [flags]"r"(flags));
+ : "=a"(rc), ASM_CALL_CONSTRAINT : [thunk_target]"r"(fop), [flags]"r"(flags));
return rc;
}