Re: [PATCH v1] LoongArch: Handle special PC in unwind_next_frame()

[Tiezhu Yang]

On 2025/11/26 下午2:08, Tiezhu Yang wrote:
> On 2025/11/25 下午6:52, Jinyang He wrote:
> > On 2025-11-25 14:33, Tiezhu Yang wrote:
> >
> > > When running virtual machine before testing the kernel live patching 
> > > with
> > > "modprobe livepatch-sample", there is a timeout over 15 seconds, the 
> > > dmesg
> > > command shows "unreliable stack" for user tasks in debug mode.
> > >
> > > The "unreliable stack" is because it can not unwind from 
> > > kvm_handle_exit()
> > > to its previous frame kvm_exc_entry() due to the PC is not a valid 
> > > kernel
> > > address, the root cause is that the code of kvm_exc_entry() was 
> > > copied to
> > > the DMW area in kvm_loongarch_env_init(), so it should check the PC 
> > > range
> > > and then finish unwinding for this special case.
> >
> > Hi, Tiezhu,
> >
> > Since kvm_loongarch_env_init copies kvm_exc_entry which similar to how
> > trap_init copies the exception handler. We should apply the same
> > offset-based adjustment to compute the shadow PC address:
> >   shadow_pc = kvm_exc_entry + (cur_pc − copied_base_address)
> > This allows the ORC unwinder to correctly locate the corresponding 
> > unwind info.
>
> Thank you. I did some tests, it can unwind from kvm_handle_exit()
> to its previous frame kvm_exc_entry() only when CONFIG_KVM=y,
> but it can not unwind from kvm_handle_exit() to its previous frame 
> kvm_exc_entry() when CONFIG_KVM=m because we can not get the two
> kvm address (unsigned long)kvm_exc_entry and
> (unsigned long)kvm_loongarch_ops->exc_entry
> in arch/loongarch/kernel/unwind_orc.c.

Use function pointer can get get the two kvm address
(unsigned long)kvm_exc_entry and
(unsigned long)kvm_loongarch_ops->exc_entry
in arch/loongarch/kernel/unwind_orc.c.

I will do more test and send v2 later.

Thanks,
Tiezhu