[kernel] Add RAII profiler object

Added profiler.h which defines classes and macros for defining profiler
objects. Also added gdb command j6prof for printing profile data. Added
the syscall_profiles profiler class and auto wrapping of syscalls with
profile objects.

Other changes in this commit:

- Made the gdb command `j6threads` argument for specifying a CPU
  optional. Without an argument, it loops through all CPUs.
- Switched to -mcmodel=kernel for kernel code, which makes `call`
  instructions easier to follow when debugging / looking at disassembly.

src/kernel/profiler.h

@@ -0,0 +1,50 @@
+#pragma once
+/// \file profiler.h
+/// The kernel profiling interface
+
+#include <stdint.h>
+#include <util/hash.h>
+
+inline uint64_t rdtsc() {
+    uint32_t high, low;
+    asm ( "rdtsc" : "=a" (low), "=d" (high) );
+    return (static_cast<uint64_t>(high) << 32) | low;
+}
+
+template<typename PC, size_t ID>
+class profiler
+{
+    using class_t = PC;
+    static constexpr size_t id = ID;
+
+public:
+    profiler(char const * const function = __builtin_FUNCTION()) : m_start(rdtsc()) {
+        static_assert(id < class_t::count, "profiler out of bounds");
+        class_t::call_counts[id]++;
+        class_t::function_names[id] = function;
+    }
+
+    ~profiler() {
+        uint64_t stop = rdtsc();
+        class_t::call_durations[id] += (stop - m_start);
+    }
+
+private:
+    uint64_t m_start;
+};
+
+template<typename T>
+struct profile_class
+{
+    static volatile char const * function_names[];
+    static volatile uint64_t call_counts[];
+    static volatile uint64_t call_durations[];
+};
+
+#define DECLARE_PROFILE_CLASS(name, size) \
+    struct name : public profile_class<name> { static constexpr size_t count = size; };
+
+#define DEFINE_PROFILE_CLASS(name) \
+    template<> volatile char const * profile_class<name>::function_names[name::count] = {0}; \
+    template<> volatile uint64_t profile_class<name>::call_counts[name::count] = {0}; \
+    template<> volatile uint64_t profile_class<name>::call_durations[name::count] = {0};

src/kernel/syscall_verify.cpp.cog

@@ -7,6 +7,7 @@
 #include <j6/types.h>
 #include <util/counted.h>
 
+#include "profiler.h"
 #include "syscalls/helpers.h"
 
 /*[[[cog code generation
@@ -21,6 +22,12 @@ for obj in syscalls.exposes:
 ]]]*/
 //[[[end]]]
 
+
+/*[[[cog code generation
+cog.outl(f'constexpr size_t syscall_count = {len(syscalls.methods)};')
+]]]*/
+//[[[end]]]
+DECLARE_PROFILE_CLASS(syscall_profiles, syscall_count);
 namespace {
     enum class req { required, optional, zero_ok };
 
@@ -61,6 +68,8 @@ namespace {
     }
 }
 
+DEFINE_PROFILE_CLASS(syscall_profiles);
+
 namespace syscalls {
 
 using util::buffer;
@@ -185,6 +194,7 @@ for id, scope, method in syscalls.methods:
     if "noreturn" in method.options:
         cog.outl(f"""    {name}({", ".join(args)});""")
     else:
+        cog.outl(f"""    profiler<syscall_profiles, {id}> profile {{"{name}"}};""")
         cog.outl(f"""    return {name}({", ".join(args)});""")
     cog.outl("}")
     cog.outl("\n")