[PAL/Linux-SGX] Simplify is_rdtsc_available logic, rely on TSC emulation

Signed-off-by: Jonathan Shamir <jsham92@gmail.com>

pal/src/host/linux-sgx/pal_exception.c

@@ -113,9 +113,11 @@ static void save_pal_context(PAL_CONTEXT* ctx, sgx_cpu_context_t* uc,
 
 #include "utils/fast_clock.h"
 
+int g_atomic_is_rdtsc_emulated = 0;
 static void emulate_rdtsc_and_print_warning(sgx_cpu_context_t* uc) {
     if (FIRST_TIME()) {
         /* if we end up emulating RDTSC/RDTSCP instruction, we cannot use TSC-based clock emulation */
+        __atomic_store_n(&g_atomic_is_rdtsc_emulated, 1, __ATOMIC_SEQ_CST);
         fast_clock_disable(&g_fast_clock);
         log_warning("all RDTSC/RDTSCP instructions are emulated (imprecisely) via gettime() "
                     "syscall.");

pal/src/host/linux-sgx/utils/fast_clock.c

@@ -191,26 +191,19 @@ static inline fast_clock_timepoint* get_timepoint(fast_clock* fast_clock, fast_c
 }
 
 static bool is_rdtsc_available(void) {
-    uint32_t words[CPUID_WORD_NUM];
+    // we just need to check if rdtsc opcode is emulated (otherwise using it is really slow) -
+    extern int g_atomic_is_rdtsc_emulated;
 
-    // rdtsc feature enabled
-    _PalCpuIdRetrieve(FEATURE_FLAGS_LEAF, 0, words);
-    if (!(words[CPUID_WORD_EDX] & (1 << 4)))
+    // "optimistic path", assume rdtsc() was called at least once
+    int is_emulated = __atomic_load_n(&g_atomic_is_rdtsc_emulated, __ATOMIC_SEQ_CST);
+    if (is_emulated) {
         return false;
+    }
 
-    // SGX enabled (sanity check)
-    _PalCpuIdRetrieve(EXTENDED_FEATURE_FLAGS_LEAF, 0, words);
-    if (!(words[CPUID_WORD_EBX] & (1 << 2)))
-        return false;
-
-    // SGX2 capabilities - otherwise, rdtsc opcode is illegal in sgx enclave
-    _PalCpuIdRetrieve(INTEL_SGX_LEAF, 0, words);
-    if (!(words[CPUID_WORD_EAX] & (1 << 1)))
-        return false;   // SGX1 features disabled
-    if (!(words[CPUID_WORD_EAX] & (1 << 2)))
-        return false;   // SGX2 features disabled
-
-    return true;
+    // make sure the is_emulated guard is initialized
+    (void)get_tsc();
+    is_emulated = __atomic_load_n(&g_atomic_is_rdtsc_emulated, __ATOMIC_SEQ_CST);
+    return !is_emulated;
 }
 
 static int handle_state_rdtsc_disabled(uint64_t* time_usec)