add sample device enumeration code

pk/device.c

@@ -0,0 +1,33 @@
+#include "pk.h"
+#include "pcr.h"
+
+static uint64_t tohost_sync(unsigned dev, unsigned cmd, uint64_t payload)
+{
+  uint64_t tohost = (uint64_t)dev << 56 | (uint64_t)cmd << 48 | payload;
+  uint64_t fromhost;
+  __sync_synchronize();
+  while (mtpcr(PCR_TOHOST, tohost));
+  while ((fromhost = mtpcr(PCR_FROMHOST, 0)) == 0);
+  __sync_synchronize();
+  return fromhost;
+}
+
+void enumerate_devices()
+{
+  char buf[64] __attribute__((aligned(64)));
+
+  for (uint64_t dev = 0; dev < 256; dev++)
+  {
+    tohost_sync(dev, 0xFF, (uintptr_t)buf << 8 | 0xFF);
+    if (buf[0])
+    {
+      printk("device %d: %s\n", dev, buf);
+      for (uint64_t cmd = 0; cmd < 255; cmd++)
+      {
+        tohost_sync(dev, 0xFF, (uintptr_t)buf << 8 | cmd);
+        if (buf[0])
+          printk("  command %d: %s\n", cmd, buf);
+      }
+    }
+  }
+}