loongarch64: able to start on legacy firmware

On legacy firmware, the DMW is already enabled by the firmware, and the
addresses in all the pointers and the PC register are virtual addresses.
GRUB, however, is location independent, so will not be affected. The
only problem happens at the GRUB_EFI_MAX_USABLE_ADDRESS, which should
be dynamically decided by detecting if DMW is enabled.

Author: shankerwangmiao

include/grub/loongarch64/efi/memory.h

@@ -19,6 +19,25 @@
 #ifndef GRUB_MEMORY_CPU_HEADER
 #include <grub/efi/memory.h>
 
-#define GRUB_EFI_MAX_USABLE_ADDRESS 0xfffffffffffULL
+static inline grub_uint64_t
+grub_efi_max_usable_address(void)
+{
+  static grub_uint64_t addr;
+  static int addr_valid = 0;
+
+  if (!addr_valid)
+  {
+    asm volatile ("csrrd %0, 0x181" : "=r" (addr));
+    if (addr & 0x1)
+      addr |= 0xfffffffffffUL;
+    else
+      addr = 0xfffffffffffUL;
+    addr_valid = 1;
+  }
+
+  return addr;
+}
+
+#define GRUB_EFI_MAX_USABLE_ADDRESS (grub_efi_max_usable_address())
 
 #endif /* ! GRUB_MEMORY_CPU_HEADER */