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

[Xiaoyao Li <xiaoyao.li@intel.com>]

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. 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.

> +       vmx->pt_desc.host_event = event;
> +       pt_load_msr(&vmx->pt_desc.guest, vmx->pt_desc.num_address_ranges);
> }
> 
>   static void pt_guest_exit(struct vcpu_vmx *vmx)
>   {
> -       if (vmx_pt_mode_is_system())
> -               return;
> +       struct perf_event *event = vmx->pt_desc.host_event;
> 
> -       if (vmx->pt_desc.guest.ctl & RTIT_CTL_TRACEEN) {
> -               pt_save_msr(&vmx->pt_desc.guest, vmx->pt_desc.num_address_ranges);
> -               pt_load_msr(&vmx->pt_desc.host, vmx->pt_desc.num_address_ranges);
> -       }
> +       if (vmx_pt_mode_is_system() ||
> +           !(vmx->pt_desc.guest.ctl & RTIT_CTL_TRACEEN))
> +               return;
> 
> -       /*
> -        * KVM requires VM_EXIT_CLEAR_IA32_RTIT_CTL to expose PT to the guest,
> -        * i.e. RTIT_CTL is always cleared on VM-Exit.  Restore it if necessary.
> -        */
> -       if (vmx->pt_desc.host.ctl)
> -               wrmsrl(MSR_IA32_RTIT_CTL, vmx->pt_desc.host.ctl);
> +       pt_save_msr(&vmx->pt_desc.guest, vmx->pt_desc.num_address_ranges);
> +       if (event)
> +               perf_event_enable(event);
>   }
> 
>   void vmx_set_host_fs_gs(struct vmcs_host_state *host, u16 fs_sel, u16 gs_sel,
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index 24d58c2ffaa3..4c20bdabc85b 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -66,7 +66,7 @@ struct pt_desc {
>          u64 ctl_bitmask;
>          u32 num_address_ranges;
>          u32 caps[PT_CPUID_REGS_NUM * PT_CPUID_LEAVES];
> -       struct pt_ctx host;
> +       struct perf_event *host_event;
>          struct pt_ctx guest;
>   };
> 
>