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

[Xiaoyao Li]

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);

Wei,

I'm not sure if we can use pt->handle.event instead or not.

> +       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