The stub has no way to ask how far RAM reaches, which is useful to
create a bitmap that covers the whole memory.

max_pfn answers that for the kernel, but it is only set in
setup_arch(), long after the stub has handed off, so, unfortunately we
cannot use it here..

Add efi_get_ram_top(), which returns the highest address reached by the
entry types that become usable RAM. Unaccepted memory is counted as
well, as suggested by Kiryl.

Memory the firmware hot-adds later is *not* in the memory map, so it is
not covered; that matches max_pfn, which is likewise the top of the
memory present at boot.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 drivers/firmware/efi/libstub/efistub.h |  2 ++
 drivers/firmware/efi/libstub/mem.c     | 53 ++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+)

diff --git a/drivers/firmware/efi/libstub/efistub.h b/drivers/firmware/efi/libstub/efistub.h
index fd91fc15ec810..77ba576779aca 100644
--- a/drivers/firmware/efi/libstub/efistub.h
+++ b/drivers/firmware/efi/libstub/efistub.h
@@ -1094,6 +1094,8 @@ char *efi_convert_cmdline(efi_loaded_image_t *image);
 efi_status_t efi_get_memory_map(struct efi_boot_memmap **map,
 				bool install_cfg_tbl);
 
+efi_status_t efi_get_ram_top(u64 *top);
+
 efi_status_t efi_allocate_pages(unsigned long size, unsigned long *addr,
 				unsigned long max);
 
diff --git a/drivers/firmware/efi/libstub/mem.c b/drivers/firmware/efi/libstub/mem.c
index 59f3f83de50c2..777f1f180c66e 100644
--- a/drivers/firmware/efi/libstub/mem.c
+++ b/drivers/firmware/efi/libstub/mem.c
@@ -64,6 +64,59 @@ efi_status_t efi_get_memory_map(struct efi_boot_memmap **map,
 	return EFI_SUCCESS;
 }
 
+/**
+ * efi_get_ram_top() - find the top of usable RAM
+ * @top:	on return, the end of the highest memory map entry that becomes
+ *		usable RAM
+ *
+ * Walk the UEFI memory map for the entry types that become usable RAM, the set
+ * setup_e820() maps to E820_TYPE_RAM, and return the highest address any of
+ * them reaches. Memory a confidential guest has not accepted yet counts as
+ * well, since it becomes RAM once accepted. This is what the stub has in place
+ * of max_pfn, which is only set once the kernel proper is up.
+ *
+ * Memory the firmware hot-adds later is not described by the memory map and so
+ * is not accounted for here.
+ *
+ * Return:	status code
+ */
+efi_status_t efi_get_ram_top(u64 *top)
+{
+	struct efi_boot_memmap *map __free(efi_pool) = NULL;
+	efi_status_t status;
+	u64 ram_top = 0;
+	int i, nr_desc;
+
+	status = efi_get_memory_map(&map, false);
+	if (status != EFI_SUCCESS)
+		return status;
+
+	nr_desc = map->map_size / map->desc_size;
+	for (i = 0; i < nr_desc; i++) {
+		efi_memory_desc_t *d;
+
+		d = efi_memdesc_ptr((unsigned long)map->map, map->desc_size, i);
+		switch (d->type) {
+		case EFI_LOADER_CODE:
+		case EFI_LOADER_DATA:
+		case EFI_BOOT_SERVICES_CODE:
+		case EFI_BOOT_SERVICES_DATA:
+		case EFI_CONVENTIONAL_MEMORY:
+		case EFI_UNACCEPTED_MEMORY:
+			ram_top = max(ram_top,
+				      d->phys_addr + d->num_pages * EFI_PAGE_SIZE);
+			break;
+		default:
+			break;
+		}
+	}
+	if (!ram_top)
+		return EFI_NOT_FOUND;
+
+	*top = ram_top;
+	return EFI_SUCCESS;
+}
+
 /**
  * efi_allocate_pages() - Allocate memory pages
  * @size:	minimum number of bytes to allocate

-- 
2.53.0-Meta