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

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Message-ID: <CAAhV-H6vxwDkBUQgY=YKnhk+3i_hW06E0UFLHK5F3VnG7tzdwA@mail.gmail.com>
Date: Sat, 27 Dec 2025 20:49:34 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Xianglai Li <lixianglai@...ngson.cn>
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>
Subject: Re: [PATCH V3 2/2] LoongArch: KVM: fix "unreliable stack" issue

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?

>
>  /*
>   * 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?

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
>