Re: [RFC PATCH 0/2] KVM: VMX: Fix VM entry failure on PT_MODE_HOST_GUEST while host is using PT

From: Liang, Kan
Date: Wed Sep 14 2022 - 16:25:46 EST



On 2022-09-14 2:16 a.m., Wang, Wei W wrote:
> On Wednesday, September 14, 2022 12:16 PM, Li, Xiaoyao wrote:
>> On 8/29/2022 3:49 PM, Wang, Wei W wrote:
>>> On Thursday, August 25, 2022 4:56 PM, Xiaoyao Li wrote:
>>>> There is one bug in KVM that can hit vm-entry failure 100% on
>>>> platform supporting PT_MODE_HOST_GUEST mode following below steps:
>>>>
>>>> 1. #modprobe -r kvm_intel
>>>> 2. #modprobe kvm_intel pt_mode=1
>>>> 3. start a VM with QEMU
>>>> 4. on host: #perf record -e intel_pt//
>>>>
>>>> The vm-entry failure happens because it violates the requirement
>>>> stated in Intel SDM 26.2.1.1 VM-Execution Control Fields
>>>>
>>>> If the logical processor is operating with Intel PT enabled (if
>>>> IA32_RTIT_CTL.TraceEn = 1) at the time of VM entry, the "load
>>>> IA32_RTIT_CTL" VM-entry control must be 0.
>>>>
>>>> On PT_MODE_HOST_GUEST node, PT_MODE_HOST_GUEST is always set.
>> Thus
>>>> KVM needs to ensure IA32_RTIT_CTL.TraceEn is 0 before VM-entry.
>>>> Currently KVM manually WRMSR(IA32_RTIT_CTL) to clear TraceEn bit.
>>>> However, it doesn't work everytime since there is a posibility that
>>>> IA32_RTIT_CTL.TraceEn is re-enabled in PT PMI handler before
>>>> vm-entry. This series tries to fix the issue by exposing two
>>>> interfaces from Intel PT driver for the purose to stop and resume
>>>> Intel PT on host. It prevents PT PMI handler from re-enabling PT. By
>>>> the way, it also fixes another issue that PT PMI touches PT MSRs whihc
>> leads to what KVM stores for host bemomes stale.
>>>
>>> I'm thinking about another approach to fixing it. I think we need to
>>> have the running host pt event disabled when we switch to guest and
>>> don't expect to receive the host pt interrupt at this point. Also, the
>>> host pt context can be save/restored by host perf core (instead of
>>> KVM) when we disable/enable the event.
>>>
>>> diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
>>> index 82ef87e9a897..1d3e03ecaf6a 100644
>>> --- a/arch/x86/events/intel/pt.c
>>> +++ b/arch/x86/events/intel/pt.c
>>> @@ -1575,6 +1575,7 @@ static void pt_event_start(struct perf_event
>>> *event, int mode)
>>>
>>> pt_config_buffer(buf);
>>> pt_config(event);
>>> + pt->event = event;
>>>
>>> return;
>>>
>>> @@ -1600,6 +1601,7 @@ static void pt_event_stop(struct perf_event
>> *event, int mode)
>>> return;
>>>
>>> event->hw.state = PERF_HES_STOPPED;
>>> + pt->event = NULL;
>>>
>>> if (mode & PERF_EF_UPDATE) {
>>> struct pt_buffer *buf = perf_get_aux(&pt->handle);
>> @@
>>> -1624,6 +1626,15 @@ static void pt_event_stop(struct perf_event *event,
>> int mode)
>>> }
>>> }
>>>
>>> +
>>> +struct perf_event *pt_get_curr_event(void) {
>>> + struct pt *pt = this_cpu_ptr(&pt_ctx);
>>> +
>>> + return pt->event;
>>> +}
>>> +EXPORT_SYMBOL_GPL(pt_get_curr_event);
>>> +
>>> static long pt_event_snapshot_aux(struct perf_event *event,
>>> struct perf_output_handle
>> *handle,
>>> unsigned long size) diff --git
>>> a/arch/x86/events/intel/pt.h b/arch/x86/events/intel/pt.h index
>>> 96906a62aacd..d46a85bb06bb 100644
>>> --- a/arch/x86/events/intel/pt.h
>>> +++ b/arch/x86/events/intel/pt.h
>>> @@ -121,6 +121,7 @@ struct pt_filters {
>>> * @output_mask: cached RTIT_OUTPUT_MASK MSR value
>>> */
>>> struct pt {
>>> + struct perf_event *event;
>>> struct perf_output_handle handle;
>>> struct pt_filters filters;
>>> int handle_nmi;
>>> diff --git a/arch/x86/include/asm/perf_event.h
>>> b/arch/x86/include/asm/perf_event.h
>>> index f6fc8dd51ef4..be8dd24922a7 100644
>>> --- a/arch/x86/include/asm/perf_event.h
>>> +++ b/arch/x86/include/asm/perf_event.h
>>> @@ -553,11 +553,14 @@ static inline int x86_perf_get_lbr(struct
>>> x86_pmu_lbr *lbr)
>>>
>>> #ifdef CONFIG_CPU_SUP_INTEL
>>> extern void intel_pt_handle_vmx(int on);
>>> + extern struct perf_event *pt_get_curr_event(void);
>>> #else
>>> static inline void intel_pt_handle_vmx(int on)
>>> {
>>>
>>> +
>>> }
>>> +struct perf_event *pt_get_curr_event(void) { }
>>> #endif
>>>
>>> #if defined(CONFIG_PERF_EVENTS) && defined(CONFIG_CPU_SUP_AMD)
>> diff
>>> --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index
>>> d7f8331d6f7e..195debc1bff1 100644
>>> --- a/arch/x86/kvm/vmx/vmx.c
>>> +++ b/arch/x86/kvm/vmx/vmx.c
>>> @@ -1125,37 +1125,29 @@ static inline void pt_save_msr(struct pt_ctx
>>> *ctx, u32 addr_range)
>>>
>>> static void pt_guest_enter(struct vcpu_vmx *vmx)
>>> {
>>> - if (vmx_pt_mode_is_system())
>>> + struct perf_event *event;
>>> +
>>> + if (vmx_pt_mode_is_system() ||
>>> + !(vmx->pt_desc.guest.ctl & RTIT_CTL_TRACEEN))
>>> return;
>>>
>>> - /*
>>> - * GUEST_IA32_RTIT_CTL is already set in the VMCS.
>>> - * Save host state before VM entry.
>>> - */
>>> - rdmsrl(MSR_IA32_RTIT_CTL, vmx->pt_desc.host.ctl);
>>> - if (vmx->pt_desc.guest.ctl & RTIT_CTL_TRACEEN) {
>>> - wrmsrl(MSR_IA32_RTIT_CTL, 0);
>>> - pt_save_msr(&vmx->pt_desc.host,
>> vmx->pt_desc.num_address_ranges);
>>> - pt_load_msr(&vmx->pt_desc.guest,
>> vmx->pt_desc.num_address_ranges);
>>> - }
>>> + event = pt_get_curr_event();
>>> + perf_event_disable(event);
>>
>> perf_event_disable() is not allowed in preemption disabled context, since
>>
>> perf_event_disable()
>> -> perf_event_ctx_lock()
>> -> perf_event_ctx_lock_nested()
>> -> mutex_lock_nested()
>>
>> and it causes
>>
>> [ 3542.164553] BUG: sleeping function called from invalid context at
>> kernel/locking/mutex.c:580
>> [ 3542.165140] in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid:
>> 1518, name: CPU 0/KVM
>> [ 3542.165703] preempt_count: 1, expected: 0 [ 3542.166006] RCU nest depth:
>> 0, expected: 0 [ 3542.166315] INFO: lockdep is turned off.
>> [ 3542.166614] irq event stamp: 0
>> [ 3542.166857] hardirqs last enabled at (0): [<0000000000000000>] 0x0
>> [ 3542.167304] hardirqs last disabled at (0): [<ffffffff94699ac8>]
>> copy_process+0x8e8/0x1bd0
>> [ 3542.167874] softirqs last enabled at (0): [<ffffffff94699ac8>]
>> copy_process+0x8e8/0x1bd0
>> [ 3542.168443] softirqs last disabled at (0): [<0000000000000000>] 0x0
>> [ 3542.168891] Preemption disabled at:
>> [ 3542.168893] [<ffffffffc0c28f29>] vcpu_enter_guest+0x139/0x1350 [kvm]
>> [ 3542.169674] CPU: 20 PID: 1518 Comm: CPU 0/KVM Not tainted
>> 6.0.0-rc5-fix-pt-vm-entry+ #3 f2d44ed9be3fc4a510291e2989c9432fce3cb5de
>> [ 3542.170457] Hardware name: Intel Corporation JACOBSVILLE/JACOBSVILLE,
>> BIOS JBVLCRB1.86B.0012.D75.1912120439 12/12/2019 [ 3542.171188] Call
>> Trace:
>> [ 3542.171392] <TASK>
>> [ 3542.171572] show_stack+0x52/0x5c
>> [ 3542.171831] dump_stack_lvl+0x5b/0x86 [ 3542.172112]
>> dump_stack+0x10/0x16 [ 3542.172371] __might_resched.cold+0x135/0x15b
>> [ 3542.172698] __might_sleep+0x52/0xa0 [ 3542.172975]
>> __mutex_lock+0x4e/0x4d0 [ 3542.173251] ? kvm_sched_in+0x4f/0x60 [kvm
>> 1f4ec973e90cea60cd68c22c1431d8e7f534c92b]
>> [ 3542.173839] ? perf_event_ctx_lock_nested+0xc8/0x230
>> [ 3542.174202] ? rcu_read_lock_sched_held+0x16/0x90
>> [ 3542.174551] ? lock_acquire+0xfc/0x150 [ 3542.174840] ?
>> perf_event_ctx_lock_nested+0x24/0x230
>> [ 3542.175205] mutex_lock_nested+0x1c/0x30 [ 3542.175505]
>> perf_event_ctx_lock_nested+0xc8/0x230
>> [ 3542.181147] ? perf_event_ctx_lock_nested+0x24/0x230
>> [ 3542.186839] perf_event_disable+0x19/0x80 [ 3542.192502]
>> vmx_vcpu_run+0x3e5/0xfe0 [kvm_intel
>> 7936a7891efe9306918aa504b0eb8bc1e7ba3aa6]
>> [ 3542.203771] ? rcu_read_lock_sched_held+0x16/0x90
>> [ 3542.209378] vcpu_enter_guest+0xa96/0x1350 [kvm
>> 1f4ec973e90cea60cd68c22c1431d8e7f534c92b]
>> [ 3542.215218] ? vcpu_enter_guest+0xbe1/0x1350 [kvm
>> 1f4ec973e90cea60cd68c22c1431d8e7f534c92b]
>> [ 3542.226292] ? rcu_read_lock_sched_held+0x16/0x90
>> [ 3542.231956] ? rcu_read_lock_sched_held+0x16/0x90
>> [ 3542.237542] ? lock_acquire+0xfc/0x150 [ 3542.243093] ?
>> __rseq_handle_notify_resume+0x3a/0x60
>> [ 3542.248689] vcpu_run+0x53/0x490 [kvm
>> 1f4ec973e90cea60cd68c22c1431d8e7f534c92b]
>> [ 3542.254533] ? vcpu_run+0x35a/0x490 [kvm
>> 1f4ec973e90cea60cd68c22c1431d8e7f534c92b]
>> [ 3542.260567] kvm_arch_vcpu_ioctl_run+0x162/0x680 [kvm
>> 1f4ec973e90cea60cd68c22c1431d8e7f534c92b]
>> [ 3542.272395] ? kvm_arch_vcpu_ioctl_run+0x6d/0x680 [kvm
>> 1f4ec973e90cea60cd68c22c1431d8e7f534c92b]
>> [ 3542.284586] kvm_vcpu_ioctl+0x2ad/0x7a0 [kvm
>> 1f4ec973e90cea60cd68c22c1431d8e7f534c92b]
>> [ 3542.290973] ? lock_acquire+0xfc/0x150 [ 3542.296990] ?
>> rcu_read_lock_sched_held+0x16/0x90
>> [ 3542.302912] ? lock_release+0x118/0x190 [ 3542.308800] ?
>> __fget_files+0xe8/0x1c0 [ 3542.314710] ? __fget_files+0x5/0x1c0
>> [ 3542.320591] __x64_sys_ioctl+0x96/0xd0 [ 3542.326500]
>> do_syscall_64+0x3a/0x90 [ 3542.332426]
>> entry_SYSCALL_64_after_hwframe+0x5e/0xc8
>>
>>
>> I know little about perf.
>
> +Kan to double confirm if needed.


The perf_event_disable() eventually invokes the intel_pt_stop().
We already expose the intel_pt_stop()/cpu_emergency_stop_pt() to other
modules. I don't think we have to use the perf_event_disable(). Also,
the perf_event_disable() requires extra codes.

I went through the discussions. I agree with Sean's suggestion.
We should only put the logic in the KVM but all the MSR access details
into the PT driver. But I prefer a more generic and straightforward
function name, e.g., intel_pt_stop_save()/intel_pt_start_load(), in case
other modules may want to save/restore the PT information in their
context switch later.

Thanks,
Kan

>
>> It seems perf_event_disable() is not used widely by
>> other kernel component. Is there a alternative? If no, I think expose
>> disable/enable helper from pt driver like this series seems OK.
>
> Oops, it was actually a mistake to disable the event on other CPUs.
> Can you expose and use perf_event_disable_local?
>
> For the enabling side, we need to add and expose perf_event_enable_local:
> event_function_local(event, __perf_event_enable, NULL);