Re: [PATCH 1/2] perf/x86/intel/pt: Fail event scheduling on conflict with VMX

[Peter Zijlstra]

On Tue, Feb 14, 2017 at 07:38:07PM +0100, Peter Zijlstra wrote:

> Right, so I question the whole 'lets not schedule PT when VMX' premise,
> it leads to inconsistencies all over. How about we treat it like
> ->add() succeeded and VMX simply results in no output.
> 
> Esp. when you then emit 'fake' data into/from a vmlaunch/vmresume
> instruction.

That is, what about something like the below? (completely untested for
obvious raisins).

That should schedule PT like normal, and where VMXON will auto-clear
TraceEn for us, we make VMXOFF set it again.

---
 arch/x86/events/intel/pt.c | 38 ++++++++++++++++++++++++++------------
 arch/x86/events/intel/pt.h |  1 +
 2 files changed, 27 insertions(+), 12 deletions(-)

diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
index 5900471ee508..a42fa1bef761 100644
--- a/arch/x86/events/intel/pt.c
+++ b/arch/x86/events/intel/pt.c
@@ -411,6 +411,7 @@ static u64 pt_config_filters(struct perf_event *event)
 
 static void pt_config(struct perf_event *event)
 {
+       struct pt *pt = this_cpu_ptr(&pt_ctx);
        u64 reg;
 
        if (!event->hw.itrace_started) {
@@ -429,11 +430,14 @@ static void pt_config(struct perf_event *event)
        reg |= (event->attr.config & PT_CONFIG_MASK);
 
        event->hw.config = reg;
-       wrmsrl(MSR_IA32_RTIT_CTL, reg);
+
+       if (!pt->vmx_on)
+               wrmsrl(MSR_IA32_RTIT_CTL, reg);
 }
 
 static void pt_config_stop(struct perf_event *event)
 {
+       struct pt *pt = this_cpu_ptr(&pt_ctx);
        u64 ctl = READ_ONCE(event->hw.config);
 
        /* may be already stopped by a PMI */
@@ -441,7 +445,9 @@ static void pt_config_stop(struct perf_event *event)
                return;
 
        ctl &= ~RTIT_CTL_TRACEEN;
-       wrmsrl(MSR_IA32_RTIT_CTL, ctl);
+
+       if (!pt->vmx_on)
+               wrmsrl(MSR_IA32_RTIT_CTL, ctl);
 
        WRITE_ONCE(event->hw.config, ctl);
 
@@ -1174,10 +1180,12 @@ void intel_pt_interrupt(void)
        /*
         * If VMX is on and PT does not support it, don't touch anything.
         */
-       if (READ_ONCE(pt->vmx_on))
+       if (READ_ONCE(pt->vmx_on)) {
+               WRITE_ONCE(pt->vmx_pmi_pending, 1);
                return;
+       }
 
-       if (!event)
+       if (WARN_ON(!event)) /* should be set if handle_nmi */
                return;
 
        pt_config_stop(event);
@@ -1236,13 +1244,22 @@ void intel_pt_handle_vmx(int on)
         */
        local_irq_save(flags);
        WRITE_ONCE(pt->vmx_on, on);
+       if (on)
+               goto done;
 
-       if (on) {
-               /* prevent pt_config_stop() from writing RTIT_CTL */
-               event = pt->handle.event;
-               if (event)
-                       event->hw.config = 0;
+       /* OTOH, if we just did VMXOFF, we need to set TraceEn again */
+       event = pt->handle.event;
+       if (!event)
+               goto done;
+
+       if (pt->vmx_pmi_pending) {
+               intel_pt_interrupt();
+               pt->vmx_pmi_pending = 0;
+       } else {
+               wrmsrl(MSR_IA32_RTIT_CTL, event->hw.config);
        }
+
+done:
        local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(intel_pt_handle_vmx);
@@ -1257,9 +1274,6 @@ static void pt_event_start(struct perf_event *event, int 
mode)
        struct pt *pt = this_cpu_ptr(&pt_ctx);
        struct pt_buffer *buf;
 
-       if (READ_ONCE(pt->vmx_on))
-               return;
-
        buf = perf_aux_output_begin(&pt->handle, event);
        if (!buf)
                goto fail_stop;
diff --git a/arch/x86/events/intel/pt.h b/arch/x86/events/intel/pt.h
index 53473c21b554..98ed385e187b 100644
--- a/arch/x86/events/intel/pt.h
+++ b/arch/x86/events/intel/pt.h
@@ -187,6 +187,7 @@ struct pt {
        struct pt_filters       filters;
        int                     handle_nmi;
        int                     vmx_on;
+       int                     vmx_pmi_pending;
 };
 
 #endif /* __INTEL_PT_H__ */