Re: [PATCH V3 2/2] LoongArch: KVM: fix "unreliable stack" issue
From: Jinyang He
Date: Sun Dec 28 2025 - 08:33:03 EST
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@xxxxxxxxxxxxxxx
Signed-off-by: Xianglai Li <lixianglai@xxxxxxxxxxx>
---
Cc: Huacai Chen <chenhuacai@xxxxxxxxxx>
Cc: WANG Xuerui <kernel@xxxxxxxxxx>
Cc: Tianrui Zhao <zhaotianrui@xxxxxxxxxxx>
Cc: Bibo Mao <maobibo@xxxxxxxxxxx>
Cc: Charlie Jenkins <charlie@xxxxxxxxxxxx>
Cc: Xianglai Li <lixianglai@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
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