KVM: nVMX: add tracepoint for failed nested VM-Enter

Debugging a failed VM-Enter is often like searching for a needle in a
haystack, e.g. there are over 80 consistency checks that funnel into
the "invalid control field" error code.  One way to expedite debug is
to run the buggy code as an L1 guest under KVM (and pray that the
failing check is detected by KVM).  However, extracting useful debug
information out of L0 KVM requires attaching a debugger to KVM and/or
modifying the source, e.g. to log which check is failing.

Make life a little less painful for VMM developers and add a tracepoint
for failed VM-Enter consistency checks.  Ideally the tracepoint would
capture both what check failed and precisely why it failed, but logging
why a checked failed is difficult to do in a generic tracepoint without
resorting to invasive techniques, e.g. generating a custom string on
failure.  That being said, for the vast majority of VM-Enter failures
the most difficult step is figuring out exactly what to look at, e.g.
figuring out which bit was incorrectly set in a control field is usually
not too painful once the guilty field as been identified.

To reach a happy medium between precision and ease of use, simply log
the code that detected a failed check, using a macro to execute the
check and log the trace event on failure.  This approach enables tracing
arbitrary code, e.g. it's not limited to function calls or specific
formats of checks, and the changes to the existing code are minimally
invasive.  A macro with a two-character name is desirable as usage of
the macro doesn't result in overly long lines or confusing alignment,
while still retaining some amount of readability.  I.e. a one-character
name is a little too terse, and a three-character name results in the
contents being passed to the macro aligning with an indented line when
the macro is used an in if-statement, e.g.:

        if (VCC(nested_vmx_check_long_line_one(...) &&
                nested_vmx_check_long_line_two(...)))
                return -EINVAL;

And that is the story of how the CC(), a.k.a. Consistency Check, macro
got its name.

Signed-off-by: Sean Christopherson <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: Sean Christopherson

arch/x86/kvm/trace.h

@@ -1475,6 +1475,24 @@ TRACE_EVENT(kvm_pv_tlb_flush,
 		__entry->need_flush_tlb ? "true" : "false")
 );
 
+/*
+ * Tracepoint for failed nested VMX VM-Enter.
+ */
+TRACE_EVENT(kvm_nested_vmenter_failed,
+	TP_PROTO(const char *msg),
+	TP_ARGS(msg),
+
+	TP_STRUCT__entry(
+		__field(const char *, msg)
+	),
+
+	TP_fast_assign(
+		__entry->msg = msg;
+	),
+
+	TP_printk("%s", __entry->msg)
+);
+
 #endif /* _TRACE_KVM_H */
 
 #undef TRACE_INCLUDE_PATH