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ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=loongson.cn; spf=pass smtp.mailfrom=loongson.cn; arc=none smtp.client-ip=114.242.206.163 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=loongson.cn Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=loongson.cn Received: from loongson.cn (unknown [111.9.175.10]) by gateway (Coremail) with SMTP id _____8CxVvDnWlJpDRAEAA--.13252S3; Mon, 29 Dec 2025 18:41:43 +0800 (CST) Received: from [10.136.12.26] (unknown [111.9.175.10]) by front1 (Coremail) with SMTP id qMiowJCx98DeWlJpRysGAA--.2622S3; Mon, 29 Dec 2025 18:41:38 +0800 (CST) Subject: Re: [PATCH V3 2/2] LoongArch: KVM: fix "unreliable stack" issue To: lixianglai Cc: loongarch@lists.linux.dev, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, stable@vger.kernel.org, Huacai Chen , WANG Xuerui , Tianrui Zhao , Bibo Mao , Charlie Jenkins , Thomas Gleixner , Tiezhu Yang References: <20251227012712.2921408-1-lixianglai@loongson.cn> <20251227012712.2921408-3-lixianglai@loongson.cn> <08143343-cb10-9376-e7df-68ad854b9275@loongson.cn> <9e1a8d4f-251f-f78e-01a3-5c483249fac8@loongson.cn> From: Jinyang He Message-ID: Date: Mon, 29 Dec 2025 18:41:33 +0800 User-Agent: Mozilla/5.0 (X11; Linux loongarch64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-CM-TRANSID:qMiowJCx98DeWlJpRysGAA--.2622S3 X-CM-SenderInfo: pkhmx0p1dqwqxorr0wxvrqhubq/ X-Coremail-Antispam: 1Uk129KBj9fXoW3uw17ur47try3Gr48Cw48AFc_yoW8JFWUJo Z093WIvw1rtF1UKF1DJwsrtr45Jw18GrnrtryUGr13Gr1Ut3WUX3yxGry7Kay5trn5Gr43 Jry3X34qyFyrtF18l-sFpf9Il3svdjkaLaAFLSUrUUUU8b8apTn2vfkv8UJUUUU8wcxFpf 9Il3svdxBIdaVrn0xqx4xG64xvF2IEw4CE5I8CrVC2j2Jv73VFW2AGmfu7bjvjm3AaLaJ3 UjIYCTnIWjp_UUUYA7kC6x804xWl14x267AKxVWUJVW8JwAFc2x0x2IEx4CE42xK8VAvwI 8IcIk0rVWrJVCq3wAFIxvE14AKwVWUXVWUAwA2ocxC64kIII0Yj41l84x0c7CEw4AK67xG Y2AK021l84ACjcxK6xIIjxv20xvE14v26r1j6r1xM28EF7xvwVC0I7IYx2IY6xkF7I0E14 v26r1j6r4UM28EF7xvwVC2z280aVAFwI0_Gr1j6F4UJwA2z4x0Y4vEx4A2jsIEc7CjxVAF wI0_Gr1j6F4UJwAaw2AFwI0_Jrv_JF1le2I262IYc4CY6c8Ij28IcVAaY2xG8wAqjxCEc2 xF0cIa020Ex4CE44I27wAqx4xG64xvF2IEw4CE5I8CrVC2j2WlYx0E2Ix0cI8IcVAFwI0_ JF0_Jw1lYx0Ex4A2jsIE14v26r4j6F4UMcvjeVCFs4IE7xkEbVWUJVW8JwACjcxG0xvEwI xGrwCYjI0SjxkI62AI1cAE67vIY487MxAIw28IcxkI7VAKI48JMxC20s026xCaFVCjc4AY 6r1j6r4UMxCIbckI1I0E14v26r1q6r43MI8I3I0E5I8CrVAFwI0_Jr0_Jr4lx2IqxVCjr7 xvwVAFwI0_JrI_JrWlx4CE17CEb7AF67AKxVWUtVW8ZwCIc40Y0x0EwIxGrwCI42IY6xII jxv20xvE14v26r1j6r1xMIIF0xvE2Ix0cI8IcVCY1x0267AKxVWUJVW8JwCI42IY6xAIw2 0EY4v20xvaj40_Jr0_JF4lIxAIcVC2z280aVAFwI0_Gr0_Cr1lIxAIcVC2z280aVCY1x02 67AKxVW8JVW8JrUvcSsGvfC2KfnxnUUI43ZEXa7IU8uuWJUUUUU== On 2025-12-29 18:11, lixianglai wrote: > Hi Jinyang: >> >> On 2025-12-29 11:53, lixianglai wrote: >>> Hi Jinyang: >>>> 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 >>>>> --- >>>>> Cc: Huacai Chen >>>>> Cc: WANG Xuerui >>>>> Cc: Tianrui Zhao >>>>> Cc: Bibo Mao >>>>> Cc: Charlie Jenkins >>>>> Cc: Xianglai Li >>>>> Cc: Thomas Gleixner >>>>> Cc: Tiezhu Yang >>>>> >>>>>   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 >>>>>   #include >>>>>   #include >>>>> +#include >>>>>     #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. >>> >>> Ok, I will use SYM_CODE_START to mark kvm_enter_guest in the next >>> version. >>> >>>> 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? >>>> >>> The stack backtracking issue in switch.S is rather complex because >>> it involves the switching between cpu root-mode and guest-mode: >>> Real stack backtracking should be divided into two parts: >>> part 1: >>>     [<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 >>> >>> part 2: >>>     [<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 >>> >>> >>> In "part 1", after executing kvm_enter_guest, the cpu switches from >>> root-mode to guest-mode. >>> In this case, stack backtracking is indeed very rare. >>> >>> In "part 2", the cpu switches from the guest-mode to the root-mode, >>> and most of the stack backtracking occurs during this phase. >>> >>> To obtain the longest call chain, we save pc in kvm_enter_guest to >>> pt_regs.csr_era, >>> and after restoring the sp of the root-mode cpu in kvm_exc_entry, >>> The ORC entry was re-established using "UNWIND_HINT_REGS", >>>  and then we obtained the following stack backtrace as we wanted: >>> >>>     [<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 >> I found this might be a coincidence—correct behavior due to the >> incorrect >> UNWIND_HINT_REGS mark and unusual stack adjustment. >> >> First, the kvm_enter_guest contains only a single branch instruction, >> ertn. >> It hardware-jump to the CSR.ERA address directly, jump into >> kvm_exc_entry. >> >> At this point, the stack layout looks like this: >> ------------------------------- >>   frame from call to `kvm_enter_guest` >> -------------------------------  <- $sp >>   PT_REGS >> -------------------------------  <- $a2 >> >> Then kvm_exc_entry adjust stack without save any register (e.g. $ra, >> $sp) >> but still marked UNWIND_HINT_REGS. >> After the adjustment: >> ------------------------------- >>   frame from call to `kvm_enter_guest` >> ------------------------------- >>   PT_REGS >> -------------------------------  <- $a2, new $sp >> >> During unwinding, when the unwinder reaches kvm_exc_entry, >> it meets the mark of PT_REGS and correctly recovers >>  pc = regs.csr_era, sp = regs.sp, ra = regs.ra >> > Yes, here unwinder does work as you say. > >> a) Can we avoid "ertn" rather than `jr reg (or jirl ra, reg, 0)` >> instead, like call? > No,  we need to rely on the 'ertn instruction return PIE to CRMD IE, > at the same time to ensure that its atomic, > there should be no other instruction than' ertn 'more appropriate here. You are right! I got it. > >> The kvm_exc_entry cannot back to kvm_enter_guest >> if we use "ertn", so should the kvm_enter_guest appear on the >> stacktrace? >> > > It is flexible. As I mentioned above, the cpu completes the switch > from host-mode to guest mode through kvm_enter_guest, > and then the switch from guest mode to host-mode through > kvm_exc_entry. When we ignore the details of the host-mode > and guest-mode switching in the middle, we can understand that the > host cpu has completed kvm_enter_guest->kvm_exc_entry. > From this perspective, I think it can exist in the call stack, and at > the same time, we have obtained the maximum call stack information. > > >> b) Can we adjust $sp before entering kvm_exc_entry? Then we can mark >> UNWIND_HINT_REGS at the beginning of kvm_exc_entry, which something >> like ret_from_kernel_thread_asm. >> > The following command can be used to dump the orc entries of the kernel: > ./tools/objtool/objtool --dump vmlinux > > You can observe that not all orc entries are generated at the > beginning of the function. > For example: > handle_tlb_protect > ftrace_stub > handle_reserved > > So, is it unnecessary for us to modify UNWIND_HINT_REGS in order to > place it at the beginning of the function. > > If you have a better solution, could you provide an example of the > modification? > I can test the feasibility of the solution. > The expression at the beginning of the function is incorrect (feeling sorry). It should be marked where have all stacktrace info. Thanks for all the explaining, since I'm unfamiliar with kvm, I need these to help my understanding. Can you try with follows, with save regs by $sp, set more precise era to pt_regs, and more unwind hint. diff --git a/arch/loongarch/kvm/switch.S b/arch/loongarch/kvm/switch.S index f1768b7a6194..8ed1d7b72c54 100644 --- a/arch/loongarch/kvm/switch.S +++ b/arch/loongarch/kvm/switch.S @@ -14,13 +14,13 @@  #define GGPR_OFFSET(x)        (KVM_ARCH_GGPR + 8*x)  .macro kvm_save_host_gpr base -    .irp n,1,2,3,22,23,24,25,26,27,28,29,30,31 +    .irp n,1,2,22,23,24,25,26,27,28,29,30,31      st.d    $r\n, \base, HGPR_OFFSET(\n)      .endr  .endm  .macro kvm_restore_host_gpr base -    .irp n,1,2,3,22,23,24,25,26,27,28,29,30,31 +    .irp n,1,2,22,23,24,25,26,27,28,29,30,31      ld.d    $r\n, \base, HGPR_OFFSET(\n)      .endr  .endm @@ -88,6 +88,7 @@      /* Load KVM_ARCH register */      ld.d    a2, a2,    (KVM_ARCH_GGPR + 8 * REG_A2) +111:      ertn /* Switch to guest: GSTAT.PGM = 1, ERRCTL.ISERR = 0, TLBRPRMD.ISTLBR = 0 */  .endm @@ -158,9 +159,10 @@ SYM_CODE_START(kvm_exc_entry)      csrwr    t0, LOONGARCH_CSR_GTLBC      ld.d    tp, a2, KVM_ARCH_HTP      ld.d    sp, a2, KVM_ARCH_HSP +    UNWIND_HINT_REGS +      /* restore per cpu register */      ld.d    u0, a2, KVM_ARCH_HPERCPU -    addi.d    sp, sp, -PT_SIZE      /* Prepare handle exception */      or    a0, s0, zero @@ -184,10 +186,11 @@ SYM_CODE_START(kvm_exc_entry)      csrwr    s1, KVM_VCPU_KS      kvm_switch_to_guest +    UNWIND_HINT_UNDEFINED  ret_to_host: -    ld.d    a2, a2, KVM_ARCH_HSP -    addi.d  a2, a2, -PT_SIZE -    kvm_restore_host_gpr    a2 +    ld.d    sp, a2, KVM_ARCH_HSP +    kvm_restore_host_gpr    sp +    addi.d    sp, sp, PT_SIZE      jr      ra  SYM_INNER_LABEL(kvm_exc_entry_end, SYM_L_LOCAL) @@ -200,11 +203,15 @@ SYM_CODE_END(kvm_exc_entry)   *  a0: kvm_run* run   *  a1: kvm_vcpu* vcpu   */ -SYM_FUNC_START(kvm_enter_guest) +SYM_CODE_START(kvm_enter_guest) +    UNWIND_HINT_UNDEFINED      /* Allocate space in stack bottom */ -    addi.d    a2, sp, -PT_SIZE +    addi.d    sp, sp, -PT_SIZE      /* Save host GPRs */ -    kvm_save_host_gpr a2 +    kvm_save_host_gpr sp +    la.pcrel a2, 111f +    st.d     a2, sp, PT_ERA +    UNWIND_HINT_REGS      addi.d    a2, a1, KVM_VCPU_ARCH      st.d    sp, a2, KVM_ARCH_HSP Jinyang