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

[lixianglai <lixianglai@loongson.cn>]

Hi Bibo Mao:
>
>
> On 2025/12/27 上午9:27, Xianglai Li 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@vger.kernel.org
>> Signed-off-by: Xianglai Li <lixianglai@loongson.cn>
>> ---
>> Cc: Huacai Chen <chenhuacai@kernel.org>
>> Cc: WANG Xuerui <kernel@xen0n.name>
>> Cc: Tianrui Zhao <zhaotianrui@loongson.cn>
>> Cc: Bibo Mao <maobibo@loongson.cn>
>> Cc: Charlie Jenkins <charlie@rivosinc.com>
>> Cc: Xianglai Li <lixianglai@loongson.cn>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Tiezhu Yang <yangtiezhu@loongson.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)
>>     /*
>>    * 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
> maybe PRMD need be set with fake pt_regs also, something like this:
>         ori     t0, zero, CSR_PRMD_PIE
>         st.d    t0, a2, PT_PRMD
>
Ok, I will fix it in the next version

Thanks,
Xianglai.
> Regards
> Bibo Mao
>> +
>>       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
>>