arm64: add the initrd region to the linear mapping explicitly

Instead of going out of our way to relocate the initrd if it turns out
to occupy memory that is not covered by the linear mapping, just add the
initrd to the linear mapping. This puts the burden on the bootloader to
pass initrd= and mem= options that are mutually consistent.

Note that, since the placement of the linear region in the PA space is
also dependent on the placement of the kernel Image, which may reside
anywhere in memory, we may still end up with a situation where the initrd
and the kernel Image are simply too far apart to be covered by the linear
region.

Since we now leave it up to the bootloader to pass the initrd in memory
that is guaranteed to be accessible by the kernel, add a mention of this to
the arm64 boot protocol specification as well.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 29 insertions, 0 deletions

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 83ae8b5e5666..82ced5fa1e66 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -207,6 +207,35 @@ void __init arm64_memblock_init(void)
                 memblock_add(__pa(_text), (u64)(_end - _text));
         }
 
+        if (IS_ENABLED(CONFIG_BLK_DEV_INITRD) && initrd_start) {
+                /*
+                 * Add back the memory we just removed if it results in the
+                 * initrd to become inaccessible via the linear mapping.
+                 * Otherwise, this is a no-op
+                 */
+                u64 base = initrd_start & PAGE_MASK;
+                u64 size = PAGE_ALIGN(initrd_end) - base;
+
+                /*
+                 * We can only add back the initrd memory if we don't end up
+                 * with more memory than we can address via the linear mapping.
+                 * It is up to the bootloader to position the kernel and the
+                 * initrd reasonably close to each other (i.e., within 32 GB of
+                 * each other) so that all granule/#levels combinations can
+                 * always access both.
+                 */
+                if (WARN(base < memblock_start_of_DRAM() ||
+                         base + size > memblock_start_of_DRAM() +
+                                       linear_region_size,
+                        "initrd not fully accessible via the linear mapping -- please check your bootloader ...\n")) {
+                        initrd_start = 0;
+                } else {
+                        memblock_remove(base, size); /* clear MEMBLOCK_ flags */
+                        memblock_add(base, size);
+                        memblock_reserve(base, size);
+                }
+        }
+
         if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {
                 extern u16 memstart_offset_seed;
                 u64 range = linear_region_size -