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

[Jinyang He <hejinyang@loongson.cn>]

On 2025-12-27 09:27, 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
Hi, Xianglai,

It should use `SYM_CODE_START` to mark the `kvm_enter_guest` rather than
`SYM_FUNC_START`, since the `SYM_FUNC_START` is used to mark "C-likely"
asm functionw. I guess the kvm_enter_guest is something like exception
handler becuase the last instruction is "ertn". So usually it should
mark UNWIND_HINT_REGS where can find last frame info by "$sp".
However, all info is store to "$a2", this mark should be
   `UNWIND_HINT sp_reg=ORC_REG_A2(???) type=UNWIND_HINT_TYPE_REGS`.
I don't konw why save this function internal PC here by `pcaddi t0, 0`,
and I think it is no meaning(, for exception handler, they save last PC
by read CSR.ERA). The `kvm_enter_guest` saves registers by
"$a2"("$sp" - PT_REGS) beyond stack ("$sp"), it is dangerous if IE
is enable. So I wonder if there is really a stacktrace through this 
function?

Jinyang


> +
>   	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