mm: Allow dynamically requesting additional memory regions

Currently, all platforms will set up their heap on initialization of the
platform code. While this works mostly fine, it poses some limitations
on memory management on us. Most notably, allocating big chunks of
memory in the gigabyte range would require us to pre-request this many
bytes from the firmware and add it to the heap from the beginning on
some platforms like EFI. As this isn't needed for most configurations,
it is inefficient and may even negatively impact some usecases when,
e.g., chainloading. Nonetheless, allocating big chunks of memory is
required sometimes, where one example is the upcoming support for the
Argon2 key derival function in LUKS2.

In order to avoid pre-allocating big chunks of memory, this commit
implements a runtime mechanism to add more pages to the system. When
a given allocation cannot be currently satisfied, we'll call a given
callback set up by the platform's own memory management subsystem,
asking it to add a memory area with at least "n" bytes. If this
succeeds, we retry searching for a valid memory region, which should
now succeed.

If this fails, we try asking for "n" bytes, possibly spread across
multiple regions, in hopes that region merging means that we end up
with enough memory for things to work out.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Tested-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Tested-by: Patrick Steinhardt <ps@pks.im>

grub-core/kern/mm.c

@@ -28,6 +28,9 @@
   - multiple regions may be used as free space. They may not be
   contiguous.
 
+  - if existing regions are insufficient to satisfy an allocation, a new
+  region can be requested from firmware.
+
   Regions are managed by a singly linked list, and the meta information is
   stored in the beginning of each region. Space after the meta information
   is used to allocate memory.
@@ -81,6 +84,7 @@
 
 
 grub_mm_region_t grub_mm_base;
+grub_mm_add_region_func_t grub_mm_add_region_fn;
 
 /* Get a header from the pointer PTR, and set *P and *R to a pointer
    to the header and a pointer to its region, respectively. PTR must
@@ -444,6 +448,32 @@ grub_memalign (grub_size_t align, grub_size_t size)
       count++;
       goto again;
 
+    case 1:
+      /* Request additional pages, contiguous */
+      count++;
+
+      if (grub_mm_add_region_fn != NULL &&
+          grub_mm_add_region_fn (size, GRUB_MM_ADD_REGION_CONSECUTIVE) == GRUB_ERR_NONE)
+	goto again;
+
+      /* fallthrough  */
+
+    case 2:
+      /* Request additional pages, anything at all */
+      count++;
+
+      if (grub_mm_add_region_fn != NULL)
+        {
+          /*
+           * Try again even if this fails, in case it was able to partially
+           * satisfy the request
+           */
+          grub_mm_add_region_fn (size, GRUB_MM_ADD_REGION_NONE);
+          goto again;
+        }
+
+      /* fallthrough */
+
     default:
       break;
     }