linux-kernel - Re: [PATCH V3 2/2] LoongArch: KVM: fix "unreliable stack" issue

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Message-ID: <18698ea8-781e-cea0-ac1f-2f0623775fac@loongson.cn>
Date: Mon, 29 Dec 2025 09:53:18 +0800
From: lixianglai <lixianglai@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org, stable@...r.kernel.org, WANG Xuerui
 <kernel@...0n.name>, Tianrui Zhao <zhaotianrui@...ngson.cn>,
 Bibo Mao <maobibo@...ngson.cn>, Charlie Jenkins <charlie@...osinc.com>,
 Thomas Gleixner <tglx@...utronix.de>, Tiezhu Yang <yangtiezhu@...ngson.cn>,
 hejinyang@...ngson.cn
Subject: Re: [PATCH V3 2/2] LoongArch: KVM: fix "unreliable stack" issue

Hi Huacai:
> Hi, Xianglai,
>
> On Sat, Dec 27, 2025 at 9:52 AM Xianglai Li <lixianglai@...ngson.cn> wrote:
>> Insert the appropriate UNWIND macro definition into the kvm_exc_entry in
>> the assembly function to guide the generation of correct ORC table entries,
>> thereby solving the timeout problem of loading the livepatch-sample module
>> on a physical machine running multiple vcpus virtual machines.
>>
>> While solving the above problems, we have gained an additional benefit,
>> that is, we can obtain more call stack information
>>
>> Stack information that can be obtained before the problem is fixed:
>> [<0>] kvm_vcpu_block+0x88/0x120 [kvm]
>> [<0>] kvm_vcpu_halt+0x68/0x580 [kvm]
>> [<0>] kvm_emu_idle+0xd4/0xf0 [kvm]
>> [<0>] kvm_handle_gspr+0x7c/0x700 [kvm]
>> [<0>] kvm_handle_exit+0x160/0x270 [kvm]
>> [<0>] kvm_exc_entry+0x100/0x1e0
>>
>> Stack information that can be obtained after the problem is fixed:
>> [<0>] kvm_vcpu_block+0x88/0x120 [kvm]
>> [<0>] kvm_vcpu_halt+0x68/0x580 [kvm]
>> [<0>] kvm_emu_idle+0xd4/0xf0 [kvm]
>> [<0>] kvm_handle_gspr+0x7c/0x700 [kvm]
>> [<0>] kvm_handle_exit+0x160/0x270 [kvm]
>> [<0>] kvm_exc_entry+0x104/0x1e4
>> [<0>] kvm_enter_guest+0x38/0x11c
>> [<0>] kvm_arch_vcpu_ioctl_run+0x26c/0x498 [kvm]
>> [<0>] kvm_vcpu_ioctl+0x200/0xcf8 [kvm]
>> [<0>] sys_ioctl+0x498/0xf00
>> [<0>] do_syscall+0x98/0x1d0
>> [<0>] handle_syscall+0xb8/0x158
>>
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Xianglai Li <lixianglai@...ngson.cn>
>> ---
>> Cc: Huacai Chen <chenhuacai@...nel.org>
>> Cc: WANG Xuerui <kernel@...0n.name>
>> Cc: Tianrui Zhao <zhaotianrui@...ngson.cn>
>> Cc: Bibo Mao <maobibo@...ngson.cn>
>> Cc: Charlie Jenkins <charlie@...osinc.com>
>> Cc: Xianglai Li <lixianglai@...ngson.cn>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Tiezhu Yang <yangtiezhu@...ngson.cn>
>>
>>   arch/loongarch/kvm/switch.S | 28 +++++++++++++++++++---------
>>   1 file changed, 19 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/loongarch/kvm/switch.S b/arch/loongarch/kvm/switch.S
>> index 93845ce53651..a3ea9567dbe5 100644
>> --- a/arch/loongarch/kvm/switch.S
>> +++ b/arch/loongarch/kvm/switch.S
>> @@ -10,6 +10,7 @@
>>   #include <asm/loongarch.h>
>>   #include <asm/regdef.h>
>>   #include <asm/unwind_hints.h>
>> +#include <linux/kvm_types.h>
>>
>>   #define HGPR_OFFSET(x)         (PT_R0 + 8*x)
>>   #define GGPR_OFFSET(x)         (KVM_ARCH_GGPR + 8*x)
>> @@ -110,9 +111,9 @@
>>           * need to copy world switch code to DMW area.
>>           */
>>          .text
>> +       .p2align PAGE_SHIFT
>>          .cfi_sections   .debug_frame
>>   SYM_CODE_START(kvm_exc_entry)
>> -       .p2align PAGE_SHIFT
>>          UNWIND_HINT_UNDEFINED
>>          csrwr   a2,   KVM_TEMP_KS
>>          csrrd   a2,   KVM_VCPU_KS
>> @@ -170,6 +171,7 @@ SYM_CODE_START(kvm_exc_entry)
>>          /* restore per cpu register */
>>          ld.d    u0, a2, KVM_ARCH_HPERCPU
>>          addi.d  sp, sp, -PT_SIZE
>> +       UNWIND_HINT_REGS
>>
>>          /* Prepare handle exception */
>>          or      a0, s0, zero
>> @@ -200,7 +202,7 @@ ret_to_host:
>>          jr      ra
>>
>>   SYM_CODE_END(kvm_exc_entry)
>> -EXPORT_SYMBOL(kvm_exc_entry)
>> +EXPORT_SYMBOL_FOR_KVM(kvm_exc_entry)
> Why not use EXPORT_SYMBOL_FOR_KVM in the first patch directly?
Ok,I will put it in the first patch.

>>   /*
>>    * int kvm_enter_guest(struct kvm_run *run, struct kvm_vcpu *vcpu)
>> @@ -215,6 +217,14 @@ SYM_FUNC_START(kvm_enter_guest)
>>          /* Save host GPRs */
>>          kvm_save_host_gpr a2
>>
>> +       /*
>> +        * The csr_era member variable of the pt_regs structure is required
>> +        * for unwinding orc to perform stack traceback, so we need to put
>> +        * pc into csr_era member variable here.
>> +        */
>> +       pcaddi  t0, 0
>> +       st.d    t0, a2, PT_ERA
> I am still confused here, does this overwrite PT_ERA stored by
> kvm_save_host_gpr?

No, kvm_save_host_gpr does not overwrite PT_ERA. In fact,
  it is precisely because PT_ERA is not assigned a value that we need to 
add these lines of statements here.

Now the a2 register points to the structure pt_regs. Here, we are 
assigning a value to the pt_regs.csr_era member variable.
Then the function unwind_next_frame uses the pt_regs.csr_era member 
variable as a pc to do the stack traceback.

Thanks,
Xianglai.

> Huacai
>
>> +
>>          addi.d  a2, a1, KVM_VCPU_ARCH
>>          st.d    sp, a2, KVM_ARCH_HSP
>>          st.d    tp, a2, KVM_ARCH_HTP
>> @@ -225,7 +235,7 @@ SYM_FUNC_START(kvm_enter_guest)
>>          csrwr   a1, KVM_VCPU_KS
>>          kvm_switch_to_guest
>>   SYM_FUNC_END(kvm_enter_guest)
>> -EXPORT_SYMBOL(kvm_enter_guest)
>> +EXPORT_SYMBOL_FOR_KVM(kvm_enter_guest)
>>
>>   SYM_FUNC_START(kvm_save_fpu)
>>          fpu_save_csr    a0 t1
>> @@ -233,7 +243,7 @@ SYM_FUNC_START(kvm_save_fpu)
>>          fpu_save_cc     a0 t1 t2
>>          jr              ra
>>   SYM_FUNC_END(kvm_save_fpu)
>> -EXPORT_SYMBOL(kvm_save_fpu)
>> +EXPORT_SYMBOL_FOR_KVM(kvm_save_fpu)
>>
>>   SYM_FUNC_START(kvm_restore_fpu)
>>          fpu_restore_double a0 t1
>> @@ -241,7 +251,7 @@ SYM_FUNC_START(kvm_restore_fpu)
>>          fpu_restore_cc     a0 t1 t2
>>          jr                 ra
>>   SYM_FUNC_END(kvm_restore_fpu)
>> -EXPORT_SYMBOL(kvm_restore_fpu)
>> +EXPORT_SYMBOL_FOR_KVM(kvm_restore_fpu)
>>
>>   #ifdef CONFIG_CPU_HAS_LSX
>>   SYM_FUNC_START(kvm_save_lsx)
>> @@ -250,7 +260,7 @@ SYM_FUNC_START(kvm_save_lsx)
>>          lsx_save_data   a0 t1
>>          jr              ra
>>   SYM_FUNC_END(kvm_save_lsx)
>> -EXPORT_SYMBOL(kvm_save_lsx)
>> +EXPORT_SYMBOL_FOR_KVM(kvm_save_lsx)
>>
>>   SYM_FUNC_START(kvm_restore_lsx)
>>          lsx_restore_data a0 t1
>> @@ -258,7 +268,7 @@ SYM_FUNC_START(kvm_restore_lsx)
>>          fpu_restore_csr  a0 t1 t2
>>          jr               ra
>>   SYM_FUNC_END(kvm_restore_lsx)
>> -EXPORT_SYMBOL(kvm_restore_lsx)
>> +EXPORT_SYMBOL_FOR_KVM(kvm_restore_lsx)
>>   #endif
>>
>>   #ifdef CONFIG_CPU_HAS_LASX
>> @@ -268,7 +278,7 @@ SYM_FUNC_START(kvm_save_lasx)
>>          lasx_save_data  a0 t1
>>          jr              ra
>>   SYM_FUNC_END(kvm_save_lasx)
>> -EXPORT_SYMBOL(kvm_save_lasx)
>> +EXPORT_SYMBOL_FOR_KVM(kvm_save_lasx)
>>
>>   SYM_FUNC_START(kvm_restore_lasx)
>>          lasx_restore_data a0 t1
>> @@ -276,7 +286,7 @@ SYM_FUNC_START(kvm_restore_lasx)
>>          fpu_restore_csr   a0 t1 t2
>>          jr                ra
>>   SYM_FUNC_END(kvm_restore_lasx)
>> -EXPORT_SYMBOL(kvm_restore_lasx)
>> +EXPORT_SYMBOL_FOR_KVM(kvm_restore_lasx)
>>   #endif
>>
>>   #ifdef CONFIG_CPU_HAS_LBT
>> --
>> 2.39.1
>>