Re: [PATCH V3 2/2] LoongArch: KVM: fix "unreliable stack" issue
From: lixianglai
Date: Sun Dec 28 2025 - 20:57:04 EST
Hi Huacai:
Hi, Xianglai,
On Sat, Dec 27, 2025 at 9:52 AM Xianglai Li <lixianglai@xxxxxxxxxxx> 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)
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