Re: [RFC PATCH] perf/kvm: Guest Symbol Resolution for powerpc

Hi David,

Thanks for the review.

On 06/16/2015 08:23 PM, David Ahern wrote:

On 6/15/15 8:50 PM, Hemant Kumar wrote:

quoted

+/*
+ * Get the instruction pointer from the tracepoint data
+ */
+u64 arch__get_ip(struct perf_evsel *evsel, struct perf_sample *data)
+{
+    u64 tp_ip = data->ip;
+    int trap;
+
+    if (!strcmp(KVMPPC_EXIT, evsel->name)) {
+        trap = raw_field_value(evsel->tp_format, "trap", 
data->raw_data);
+
+        if (trap == HV_DECREMENTER)
+            tp_ip = raw_field_value(evsel->tp_format, "pc",
+                        data->raw_data);
+    }
+    return tp_ip;
+}

You can tie a handler to an event; see builtin-trace.c for example 
(evsel->handler = handler). Then have the sample handler call it (e.g, 
see trace__process_sample). Then you don't have to check event names 
on each pass like this and just do event based processing.

quoted

+
+/*
+ * Get the HV and PR bits and accordingly, determine the cpumode
+ */
+u8 arch__get_cpumode(union perf_event *event, struct perf_evsel *evsel,
+             struct perf_sample *data)
+{
+    unsigned long hv, pr, msr;
+    u8 cpumode = event->header.misc & PERF_RECORD_MISC_CPUMODE_MASK;
+
+    if (strcmp(KVMPPC_EXIT, evsel->name))
+        goto ret;
+
+    if (data->raw_data)
+        msr = raw_field_value(evsel->tp_format, "msr", data->raw_data);
+    else
+        goto ret;
+
+    hv = msr & ((long unsigned)1 << (PPC_MAX - HV_BIT));
+    pr = msr & ((long unsigned)1 << (PPC_MAX - PR_BIT));
+
+    if (!hv && pr)
+        cpumode = PERF_RECORD_MISC_GUEST_USER;
+    else
+        cpumode = PERF_RECORD_MISC_GUEST_KERNEL;
+ret:
+    return cpumode;
+}

Why isn't that set properly kernel side when the sample is generated?

Because, this depends on the kernel tracepoint "kvm_hv:kvm_guest_exit".
perf_prepare_sample() in the kernel side sets the event->header.misc 
field to
PERF_RECORD_MISC_KERNEL through perf_misc_flags(pt_regs). In case of
tracepoints which always get hit in the host kernel context, the
perf_misc_flags() will always return PERF_RECORD_MISC_KERNEL.

IMHO we will rather have to set the cpumode in the user space for this 
tracepoint
and we can't depend on the event->header.misc field for this case.

What would you suggest?

-- 
Thanks,
Hemant Kumar