linux-kernel - Re: [PATCH v1] LoongArch: Handle special PC in unwind_next

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Message-ID: <6cc59312-7cdc-19a5-4d18-e120ea9c7563@loongson.cn>
Date: Wed, 26 Nov 2025 14:08:27 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Jinyang He <hejinyang@...ngson.cn>
Cc: Huacai Chen <chenhuacai@...nel.org>, Xi Zhang <zhangxi@...inos.cn>,
 Xianglai Li <lixianglai@...ngson.cn>, loongarch@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] LoongArch: Handle special PC in unwind_next_frame()

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.

The KVM team will provide a patch to do something, then I will
test it again.

Thanks,
Tiezhu