在 2024/9/6 17:39, Mark Rutland 写道: > On Tue, Aug 27, 2024 at 07:33:55PM +0800, Liao, Chang wrote: >> Hi, Mark >> >> Would you like to discuss this patch further, or do you still believe emulating >> STP to push FP/LR into the stack in kernel is not a good idea? > > I'm happy with the NOP emulation in principle, so please send a new > version with *just* the NOP emulation, and I can review that. Mark, please check v2 from link: https://lore.kernel.org/all/20240909071114.1150053-1-liaochang1@xxxxxxxxxx/ Thanks. > > Regarding STP emulation, I stand by my earlier comments, and in addition > to those comments, AFAICT it's currently unsafe to use any uaccess > routine in the uprobe BRK handler anyway, so that's moot. The uprobe BRK > handler runs with preemption disabled and IRQs (and all other maskable > exceptions) masked, and faults cannot be handled. IIUC > CONFIG_DEBUG_ATOMIC_SLEEP should scream about that. > > Longer-term I'm happy to look into making it possible to do uaccesses in > the uprobe handlers, but that requires a much wider rework of the way we > handle BRK instructions, single-step, and hardware breakpoints and > watchpoints. > > Mark. > >> Thanks. >> >> >> 在 2024/8/21 15:55, Liao, Chang 写道: >>> Hi, Mark >>> >>> My bad for taking so long to rely, I generally agree with your suggestions to >>> STP emulation. >>> >>> 在 2024/8/15 17:58, Mark Rutland 写道: >>>> On Wed, Aug 14, 2024 at 08:03:56AM +0000, Liao Chang wrote: >>>>> As Andrii pointed out, the uprobe/uretprobe selftest bench run into a >>>>> counterintuitive result that nop and push variants are much slower than >>>>> ret variant [0]. The root cause lies in the arch_probe_analyse_insn(), >>>>> which excludes 'nop' and 'stp' from the emulatable instructions list. >>>>> This force the kernel returns to userspace and execute them out-of-line, >>>>> then trapping back to kernel for running uprobe callback functions. This >>>>> leads to a significant performance overhead compared to 'ret' variant, >>>>> which is already emulated. >>>> >>>> I appreciate this might be surprising, but does it actually matter >>>> outside of a microbenchmark? >>> >>> I just do a simple comparsion the performance impact of single-stepped and >>> emulated STP on my local machine. Three user cases were measured: Redis GET and >>> SET throughput (Request Per Second, RPS), and the time taken to execute a grep >>> command on the "arch_uprobe_copy_xol" string within the kernel source. >>> >>> Redis GET (higher is better) >>> ---------------------------- >>> No uprobe: 49149.71 RPS >>> Single-stepped STP: 46750.82 RPS >>> Emulated STP: 48981.19 RPS >>> >>> Redis SET (larger is better) >>> ---------------------------- >>> No uprobe: 49761.14 RPS >>> Single-stepped STP: 45255.01 RPS >>> Emulated stp: 48619.21 RPS >>> >>> Grep (lower is better) >>> ---------------------- >>> No uprobe: 2.165s >>> Single-stepped STP: 15.314s >>> Emualted STP: 2.216s >>> >>> The result reveals single-stepped STP instruction that used to push fp/lr into >>> stack significantly impacts the Redis and grep performance, leading to a notable >>> notable decrease RPS and increase time individually. So emulating STP on the >>> function entry might be a more viable option for uprobe. >>> >>>> >>>>> Typicall uprobe is installed on 'nop' for USDT and on function entry >>>>> which starts with the instrucion 'stp x29, x30, [sp, #imm]!' to push lr >>>>> and fp into stack regardless kernel or userspace binary. >>>> >>>> Function entry doesn't always start with a STP; these days it's often a >>>> BTI or PACIASP, and for non-leaf functions (or with shrink-wrapping in >>>> the compiler), it could be any arbitrary instruction. This might happen >>>> to be the common case today, but there are certain;y codebases where it >>>> is not. >>> >>> Sure, if kernel, CPU and compiler support BTI and PAC, the entry instruction >>> is definitly not STP. But for CPU and kernel lack of these supports, STP as >>> the entry instruction is still the common case. And I profiled the entry >>> instruction for all leaf and non-leaf function, the ratio of STP is 64.5% >>> for redis, 76.1% for the BPF selftest bench. So I am thinking it is still >>> useful to emulate the STP on the function entry. Perhaps, for CPU and kernel >>> with BTI and PAC enabled, uprobe chooses the slower single-stepping to execute >>> STP for pushing stack. >>> >>>> >>>> STP (or any instruction that accesses memory) is fairly painful to >>>> emulate because you need to ensure that the correct atomicity and >>>> ordering properties are provided (e.g. an aligned STP should be >>>> single-copy-atomic, but copy_to_user() doesn't guarantee that except by >>>> chance), and that the correct VMSA behaviour is provided (e.g. when >>>> interacting with MTE, POE, etc, while the uaccess primitives don't try >>>> to be 100% equivalent to instructions in userspace). >>> Agreed, but I don't think it has to emulate strictly the single-copy-atomic >>> feature of STP that is used to push fp/lr into stack. In most cases, only one >>> CPU will push registers to the same position on stack. And I barely understand >>> why other CPUs would depends on the ordering of pushing data into stack. So it >>> means the atomicity and ordering is not so important for this scenario. Regarding >>> MTE and POE, a similar stragety to BTI and PAC can be applied: for CPUs and kernel >>> with MTE and POE enabled, uprobe chooses the slower single-stepping to execute >>> STP for pushing stack. >>> >>>> >>>> For those reasons, in general I don't think we should be emulating any >>>> instruction which accesses memory, and we should not try to emulate the >>>> STP, but I think it's entirely reasonable to emulate NOP. >>>> >>>>> In order to >>>>> improve the performance of handling uprobe for common usecases. This >>>>> patch supports the emulation of Arm64 equvialents instructions of 'nop' >>>>> and 'push'. The benchmark results below indicates the performance gain >>>>> of emulation is obvious. >>>>> >>>>> On Kunpeng916 (Hi1616), 4 NUMA nodes, 64 Arm64 cores@2.4GHz. >>>>> >>>>> xol (1 cpus) >>>>> ------------ >>>>> uprobe-nop: 0.916 ± 0.001M/s (0.916M/prod) >>>>> uprobe-push: 0.908 ± 0.001M/s (0.908M/prod) >>>>> uprobe-ret: 1.855 ± 0.000M/s (1.855M/prod) >>>>> uretprobe-nop: 0.640 ± 0.000M/s (0.640M/prod) >>>>> uretprobe-push: 0.633 ± 0.001M/s (0.633M/prod) >>>>> uretprobe-ret: 0.978 ± 0.003M/s (0.978M/prod) >>>>> >>>>> emulation (1 cpus) >>>>> ------------------- >>>>> uprobe-nop: 1.862 ± 0.002M/s (1.862M/prod) >>>>> uprobe-push: 1.743 ± 0.006M/s (1.743M/prod) >>>>> uprobe-ret: 1.840 ± 0.001M/s (1.840M/prod) >>>>> uretprobe-nop: 0.964 ± 0.004M/s (0.964M/prod) >>>>> uretprobe-push: 0.936 ± 0.004M/s (0.936M/prod) >>>>> uretprobe-ret: 0.940 ± 0.001M/s (0.940M/prod) >>>>> >>>>> As shown above, the performance gap between 'nop/push' and 'ret' >>>>> variants has been significantly reduced. Due to the emulation of 'push' >>>>> instruction needs to access userspace memory, it spent more cycles than >>>>> the other. >>>>> >>>>> [0] https://lore.kernel.org/all/CAEf4BzaO4eG6hr2hzXYpn+7Uer4chS0R99zLn02ezZ5YruVuQw@xxxxxxxxxxxxxx/ >>>>> >>>>> Signed-off-by: Liao Chang <liaochang1@xxxxxxxxxx> >>>>> --- >>>>> arch/arm64/include/asm/insn.h | 21 ++++++++++++++++++ >>>>> arch/arm64/kernel/probes/decode-insn.c | 18 +++++++++++++-- >>>>> arch/arm64/kernel/probes/decode-insn.h | 3 ++- >>>>> arch/arm64/kernel/probes/simulate-insn.c | 28 ++++++++++++++++++++++++ >>>>> arch/arm64/kernel/probes/simulate-insn.h | 2 ++ >>>>> arch/arm64/kernel/probes/uprobes.c | 2 +- >>>>> 6 files changed, 70 insertions(+), 4 deletions(-) >>>>> >>>>> diff --git a/arch/arm64/include/asm/insn.h b/arch/arm64/include/asm/insn.h >>>>> index 8c0a36f72d6f..a246e6e550ba 100644 >>>>> --- a/arch/arm64/include/asm/insn.h >>>>> +++ b/arch/arm64/include/asm/insn.h >>>>> @@ -549,6 +549,27 @@ static __always_inline bool aarch64_insn_uses_literal(u32 insn) >>>>> aarch64_insn_is_prfm_lit(insn); >>>>> } >>>>> >>>>> +static __always_inline bool aarch64_insn_is_nop(u32 insn) >>>>> +{ >>>>> + /* nop */ >>>>> + return aarch64_insn_is_hint(insn) && >>>>> + ((insn & 0xFE0) == AARCH64_INSN_HINT_NOP); >>>>> +} >>>> >>>> This looks fine, but the comment can go. >>> >>> Removed. >>> >>>> >>>>> +static __always_inline bool aarch64_insn_is_stp_fp_lr_sp_64b(u32 insn) >>>>> +{ >>>>> + /* >>>>> + * The 1st instruction on function entry often follows the >>>>> + * patten 'stp x29, x30, [sp, #imm]!' that pushing fp and lr >>>>> + * into stack. >>>>> + */ >>>>> + return aarch64_insn_is_stp_pre(insn) && >>>>> + (((insn >> 30) & 0x03) == 2) && /* opc == 10 */ >>>>> + (((insn >> 5) & 0x1F) == 31) && /* Rn is sp */ >>>>> + (((insn >> 10) & 0x1F) == 30) && /* Rt2 is x29 */ >>>>> + (((insn >> 0) & 0x1F) == 29); /* Rt is x30 */ >>>>> +} >>>> >>>> We have accessors for these fields. Please use them. >>> >>> Do you mean aarch64_insn_decode_register()? >>> >>>> >>>> Regardless, as above I do not think we should have a helper this >>>> specific (with Rn, Rt, and Rt2 values hard-coded) inside <asm/insn.h>. >>> >>> If we left necessary of emulation of STP aside, where would the best file to >>> place these hard-coded decoder helper? >>> >>>> >>>>> enum aarch64_insn_encoding_class aarch64_get_insn_class(u32 insn); >>>>> u64 aarch64_insn_decode_immediate(enum aarch64_insn_imm_type type, u32 insn); >>>>> u32 aarch64_insn_encode_immediate(enum aarch64_insn_imm_type type, >>>>> diff --git a/arch/arm64/kernel/probes/decode-insn.c b/arch/arm64/kernel/probes/decode-insn.c >>>>> index 968d5fffe233..df7ca16fc763 100644 >>>>> --- a/arch/arm64/kernel/probes/decode-insn.c >>>>> +++ b/arch/arm64/kernel/probes/decode-insn.c >>>>> @@ -73,8 +73,22 @@ static bool __kprobes aarch64_insn_is_steppable(u32 insn) >>>>> * INSN_GOOD_NO_SLOT If instruction is supported but doesn't use its slot. >>>>> */ >>>>> enum probe_insn __kprobes >>>>> -arm_probe_decode_insn(probe_opcode_t insn, struct arch_probe_insn *api) >>>>> +arm_probe_decode_insn(probe_opcode_t insn, struct arch_probe_insn *api, >>>>> + bool kernel) >>>>> { >>>>> + /* >>>>> + * While 'nop' and 'stp x29, x30, [sp, #imm]! instructions can >>>>> + * execute in the out-of-line slot, simulating them in breakpoint >>>>> + * handling offers better performance. >>>>> + */ >>>>> + if (aarch64_insn_is_nop(insn)) { >>>>> + api->handler = simulate_nop; >>>>> + return INSN_GOOD_NO_SLOT; >>>>> + } else if (!kernel && aarch64_insn_is_stp_fp_lr_sp_64b(insn)) { >>>>> + api->handler = simulate_stp_fp_lr_sp_64b; >>>>> + return INSN_GOOD_NO_SLOT; >>>>> + } >>>> >>>> With the STP emulation gone, you won't need the kernel parameter here.> >>>>> + >>>>> /* >>>>> * Instructions reading or modifying the PC won't work from the XOL >>>>> * slot. >>>>> @@ -157,7 +171,7 @@ arm_kprobe_decode_insn(kprobe_opcode_t *addr, struct arch_specific_insn *asi) >>>>> else >>>>> scan_end = addr - MAX_ATOMIC_CONTEXT_SIZE; >>>>> } >>>>> - decoded = arm_probe_decode_insn(insn, &asi->api); >>>>> + decoded = arm_probe_decode_insn(insn, &asi->api, true); >>>>> >>>>> if (decoded != INSN_REJECTED && scan_end) >>>>> if (is_probed_address_atomic(addr - 1, scan_end)) >>>>> diff --git a/arch/arm64/kernel/probes/decode-insn.h b/arch/arm64/kernel/probes/decode-insn.h >>>>> index 8b758c5a2062..ec4607189933 100644 >>>>> --- a/arch/arm64/kernel/probes/decode-insn.h >>>>> +++ b/arch/arm64/kernel/probes/decode-insn.h >>>>> @@ -28,6 +28,7 @@ enum probe_insn __kprobes >>>>> arm_kprobe_decode_insn(kprobe_opcode_t *addr, struct arch_specific_insn *asi); >>>>> #endif >>>>> enum probe_insn __kprobes >>>>> -arm_probe_decode_insn(probe_opcode_t insn, struct arch_probe_insn *asi); >>>>> +arm_probe_decode_insn(probe_opcode_t insn, struct arch_probe_insn *asi, >>>>> + bool kernel); >>>>> >>>>> #endif /* _ARM_KERNEL_KPROBES_ARM64_H */ >>>>> diff --git a/arch/arm64/kernel/probes/simulate-insn.c b/arch/arm64/kernel/probes/simulate-insn.c >>>>> index 22d0b3252476..0b1623fa7003 100644 >>>>> --- a/arch/arm64/kernel/probes/simulate-insn.c >>>>> +++ b/arch/arm64/kernel/probes/simulate-insn.c >>>>> @@ -200,3 +200,31 @@ simulate_ldrsw_literal(u32 opcode, long addr, struct pt_regs *regs) >>>>> >>>>> instruction_pointer_set(regs, instruction_pointer(regs) + 4); >>>>> } >>>>> + >>>>> +void __kprobes >>>>> +simulate_nop(u32 opcode, long addr, struct pt_regs *regs) >>>>> +{ >>>>> + instruction_pointer_set(regs, instruction_pointer(regs) + 4); >>>>> +} >>>> >>>> Hmm, this forgets to update the single-step state machine and PSTATE.BT, >>>> and that's an extant bug in arch_uprobe_post_xol(). This can be: >>> >>> For emulated instruction, uprobe won't enable single-step mode of CPU, >>> please check the handle_swbp() in kernel/events/uprobes.c: >>> >>> if (arch_uprobe_skip_sstep(&uprobe->arch, regs)) >>> goto out; >>> >>> if (!pre_ssout(uprobe, regs, bp_vaddr)) >>> return; >>> >>> For emualted instruction, It will skip entire single-stepping and associated >>> exception, which typically begins with pre_ssout() and ends with >>> arch_uprobe_post_xol(). Therefore, using instruction_pointer_set() to emulate >>> NOP is generally not a bad idea. >>> >>>> >>>> | void __kprobes >>>> | simulate_nop(u32 opcode, long addr, struct pt_regs *regs) >>>> | { >>>> | arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE); >>>> | } >>>> >>>>> + >>>>> +void __kprobes >>>>> +simulate_stp_fp_lr_sp_64b(u32 opcode, long addr, struct pt_regs *regs) >>>>> +{ >>>>> + long imm7; >>>>> + u64 buf[2]; >>>>> + long new_sp; >>>>> + >>>>> + imm7 = sign_extend64((opcode >> 15) & 0x7f, 6); >>>>> + new_sp = regs->sp + (imm7 << 3); >>>> >>>> We have accessors for these fields, please use them. >>> >>> Do you mean aarch64_insn_decode_immediate()? >>> >>>> >>>>> + >>>>> + buf[0] = regs->regs[29]; >>>>> + buf[1] = regs->regs[30]; >>>>> + >>>>> + if (copy_to_user((void __user *)new_sp, buf, sizeof(buf))) { >>>>> + force_sig(SIGSEGV); >>>>> + return; >>>>> + } >>>> >>>> As above, this won't interact with VMSA features (e.g. MTE, POE) in the >>>> same way as an STP in userspace, and this will not have the same >>>> atomicity properties as an STP> >>>>> + >>>>> + regs->sp = new_sp; >>>>> + instruction_pointer_set(regs, instruction_pointer(regs) + 4); >>>> >>>> Likewise, this sould need ot use arm64_skip_faulting_instruction(), >>>> though as above I think we should drop STP emulation entirely. >>> >>> I explain the reason why using instruction_pointer_set() under your comments >>> for simulate_nop(). >>> >>> Thanks. >>> >>>> >>>> Mark. >>>> >>> >> >> -- >> BR >> Liao, Chang > -- BR Liao, Chang
